Warning: Spoilers ahead.
Most of these characters and events had not appeared in the anime yet.
Every shounen anime series has a long list of characters that can get confusing. Demon Slayer (Kimetsu no Yaiba) is no stranger to that. Upon the introduction of the anime, there were already characters that had made a mark on viewers.
What makes it great is that certain groups of people within the anime make people ask who they are. When Hashiras or Pillars were mentioned, former Hashiras, Upper-Rank Moon, or Lower Rank Moon, also known as the Twelve Kizuki, these and many others had fans craving more information.
It is not just the character design of characters that made them extraordinary but the type of characters they are. Fans tend to be selective with the characters they are rooting for. Some would want the fun-loving and free-spirited characters, while some would go for the cold tsundere type.
All shounen anime got to have a mentor, a great teacher who will guide the main protagonist in their journey. Some of these mentors appear in the middle of the series when the main protagonist is experiencing a slump, while others would appear from the beginning. Whichever it is, who is your favorite Demon Slayer character?
The 4 Main Characters
Tanjiro Kamado

The main protagonist of the series. He has a strong sense of smell and uses the Breath of Water style. Later in the manga, he started using the Breath of Sun style. The story revolves around his journey to avenge his family, killed by the leader of the demon race, Muzan. His sense of justice is equal to none, and he has one of the purest hearts.
Nezuko Kamado

The sister of Tanjiro survived the massacre of their family. She was turned into a demon and was almost killed by Giyu in their first encounter. She was a demon, but she was put in hypnosis to not harm any human. Like any other demon, she possesses a Demon Blood Art, but she uses the art to assist her brother in his fight against the demons.
Zenitsu Agatsuma

The heir to the Breath of Thunder, Zenitsu is a timid boy who uses his full potential when asleep. He has a huge crush on Nezuko. Zenitsu has low self-esteem. He is a coward and is not aware of the strength that he possesses.
Inosuke Hashibira

Inosuke is an orphan who a boar had raised. His notable feature is wearing the boar mask, which belongs to his adoptive boar mother. He is vengeful and wants to be the strongest man in the room. He wants to challenge the people who are stronger than him. In the Infinity Castle Arc, he challenged Doma, who ate Shinobu, and later challenged Muzan to a one-on-one fight because Muzan decapitated and killed a lot of Demon Slayers.
Demon Slayer Corps
This organization is made up of the strongest swordsmen all over Japan. These swordsmen swore allegiance to the Ubuyashiki family. They vow to fight demons and put their life on the line. Members of the Corps are each given areas to patrol. They are to gather information and improve their skills.
Each member has their signature breathing style. Fire Breathing and Water Breathing user always appears in every generation of Hashira, and a former Hashira always mentors them with the same breathing style.
Joining the Corps is no easy feat. It is not for the faint-hearted. Children as young as 12 need to be trained by a former Hashira or other Corps members to learn the technique of killing demons. The exam includes getting through traps, surviving the night surrounded by demons, and killing a demon.
The preparation for the exam and taking the exam was shown in the first few episodes of the anime, where Tanjiro Kamado spent two years training to pass the exam.
To become the Hashira, a member of the Corps needs to kill at least 50 demons, a Kizuki, or a member of the 12 highest rank members of the demon race. Another method to become a Hashira is to be the Tsuguko of a Hashira. This means you need to be a successor to a Hashira.
To be considered a successor, you need to be one of the most talented in your generation. Becoming a Tsuguko is done either through application or selection.
Some Hashira would already choose the people they want to replace them, and most of them are the younger members with the same breathing style. The similarity in breathing style is not necessary; most Hashira would choose those with the same breathing style because it is easier for them to train those people, and they can guide them easily.
Leader of Demon Slayer Corps
Kagaya Ubuyashiki

The 97th leader of the Corps. He comes from a family that offered their life to defeat the ultimate demon, Muzan. The Ubuyashiki family is not meant to live over 30. He met his wife when he was only 13 and Amane was 17. Since priests choose wives in the Ubuyashiki family, he did not agree to the marriage until Amane agreed that she would care for him of her own will. He was later replaced by his only son and middle child Kiriya Ubuyashiki, who became the last leader of the Corps.
Hashira
Giyu Tomioka

The Water Hashira. He trained under a former Hashira Sakonji Urokodaki, who later mentored Tanjiro. Giyu suffers from an inferiority complex after he passes the Demon Slayer Corps exam after being saved by Sabito. He developed an inferiority complex, and he doubts himself about what he can do since his sister also died saving him from a demon.
Mitsuri Kanroji

She is the Love Hashira and initially joined the Corps to find a husband who is stronger than her. This isn’t easy, given that she is one of the strongest in the Corps. Mitsuri is the kindest and most emotional among the Hashira. Despite being emotional, she does not condone evil, and she could be the worst nightmare for a demon. Most of her attacks are named after the cat because she had cats when she was a child.
Obanai Iguro

He is the Serpent Hashira, known to be brutal and rude towards those he deems weaker than him. He is known to be strict and harsh. He does not tolerate those that do not follow the rules of the Demon Slayer Corps. Despite his merciless personality, he always shows how much he cares for Mitsuri. He even showers her with gifts and shares meals with her. The green socks that Mitsuri is wearing are a gift from him.
Sanemi Shinazugawa
This Wind Hashira is known for being hot-blooded, abrasive, and quick to lash out. He is the older brother of Genya Shinazugawa. He never got along with any of the Hashira, but in the showdown with Muzan, he cooperated with Giyu. He lost the people most dear to him to demons. This is why he hated demons so much that when he first met the Kamado siblings, he never believed Nezuko would ever harm humans.
Gyomei Himejima
This Stone Hashira is also known as the gentle giant. He has a large physique, yet he is soft-spoken and sensitive. He is also considered the leader of Hashira. His upbringing makes him care for all living creatures. He cares for demons and gets affected when he has to beat up a demon. Despite being sensitive, he is always doubtful of others and will take time before he trusts others.
Tengen Uzui
This Sound Hashira calls himself the god of flashiness. He wants everything to be flashy, from his clothes, headdress, and everything about him. Tengen has three wives, Makio, Suma, and Hinatsuru, whom he deeply cares for. He looks down on others for not being as flashy as him, and he always encourages those around him to praise and humor him. He is known for his stamina and would always be indifferent to those who cannot complete their stamina training with him.
Muichiro Tokito
This Mist Hashira does not work with others. He always let his logic guide him. To most people, he is emotionless, and he always lets his mind wander around. The “Mu” in his name means infinity, as revealed by his twin brother Yuichiro who died in a demon attack. Amane recruited him and his brother, but Yuichiro chased her away until they got attacked by the demon one night and Yuichiro died. When his twin brother died, Muichiro became cod opposite to his past personality, which is compassionate and kind.
Shinobu Kocho
This Insect Hashira is known for her bright and cheery personality. She always wears a smile regardless of the situation. She likes teasing others and picking on them. She held a deep grudge toward demons when she lost the people she loved because of them. The attack on the Butterfly Mansion killed almost everyone, including her apprentice’s family. Her parents also died, and most of her Tsuguko also died. Now the Butterfly Mansion is used as a recovery area for injured members of the Corps.
Kyojuro Rengoku
This Flame Hashira is known for his eccentric personality. He grows up in a Hashira household, his father being a former Flame Hashira. Despite having a strict father, he still grew up being loved and valued. He greatly loves his family and has high respect for his father. He also shows how he values the lives of others and would always be calculated in the battle to ensure that the lives of the people involved would not be sacrificed. Kyojuro is known to be a highly-intelligent Hashira and a tactician in battle.
Former Hashira
Yoriichi Tsugikuni
One the first Hashira. He was the one who almost defeated Muzan, and he wore the Hanafuda earrings. He does not have descendants, so he gave the Hanafuda earrings to his friend Sumiyoshi who handed them down to generations until Tanjiro got hold of it. He also taught a “dance” to Sumiyoshi; later, that dance was the Breath of Sun style. Yoriichi is the first Breath of Sun user and twin of the Upper Demon Rank 1 Kokushibo.
Kanae Kocho

She is a former Flower Hashira that was slain years ago. This happened when he encountered Upper Moon 2, Doma. Her body was not eaten as Doma had to flee from the rising sun. When Kanae was dying in Shinobu’s arms, she told her to leave the Demon Slayer Corps and live a simple life.
Sakonji Urokodaki

The former Water Hashira. He was the mentor of Giyu and Tanjiro. He is known for wearing a red mask and lives in the mountain alone. Not much of his past is known, nor for how long he had been a Water Hashira. Sakonji was the one who captured the hand demon and had proven his place as a Hashira during his time. In the Infinity Castle Arc, he was the one who was summoned to the Butterfly Mansion to protect Nezuko from Muzan.
Jigoro Kuwajima

The former Thunder Hashira is the mentor of Zenitsu Agatsuma and Kaigaku. He had always wanted Zenitsu to be his successor. When Kaigaku became a demon, Jigoro committed seppuku. He met Zenitsu during the Infinity Castle Arc after Zenitsu killed Kaigaku. Zenitsu asked for forgiveness from Jigoro for not being able to get along with Kaigaku. Jigoro told Zenitsu that he was his pride and joy.
Shinjuro Renguku

This former Flame Hashira is the father of Kyojuro Renguku and bears his resemblance. He began drinking after his wife died. He drinks while he does Hashira duty, much to the dismay of many. He saved Oganai from a Serpent Demon. His personality changed when his wife died. He became short-tempered with a mention of Demon Slayer Corps. When his son, Kyojuro, became a Hashira, he told him that it was not something to be proud of.
Tsuguko
Kanao Tsuyuri
She is the only known Tsuguko in the series. She has the Flower Breathing ability. She is from the same generation as Tanjiro and took the Demon Slayer Corps exam alongside Tanjiro, Zenitsu, and Inosuke. She is also the adopted sister of Shinobu and Kanae. She was not taught the Flower Breathing Style. Instead, she learned it while observing her big sister Kanae.
Kakushi
They are part of the brigade that does other tasks such as serving as attendants, cleaning the mess after a huge battle, and all that is related to cleaning services. They also sew and repair the clothes of the member of the Corps.
Goto

He is a notable member of the Kakushi and is at one point found worthy of being a demon slayer, but he opts to become Kakushi.
Ubuyashiki Family
Amane Ubuyashikie – Mother
Hinaki Ubuyashiki – Daughter
Kanata Ubuyashiki – Daughter
Kiriya Ubuyashiki – Son and would later replace his father as the last leader of the Corps.
Kuina Ubuyashiki – Daughter
Michika Ubuyashiki – Daughter
Demons
These are known to be the fiercest and the strongest among the demons. The difference in power between the two had been immense. The lower-rank demons are always changing since they could easily be defeated by Hashiras, while the upper rank has not changed over the years.
The upper-rank demons are known to have eaten countless humans, including Hashiras. If a member is not satisfied with the ranking, they are free to challenge the one in the higher rank. If the one in the higher rank is defeated, then there will be changes in ranking.
When the lower ranks keep changing, Muzan has to continuously assign a new demon to the lower rank. Finding this tedious, he decided to dismantle the lower-rank demons and kill all of them except for Enmu, who was given the last chance and was even given an added injection of the blood of Muzan.
All the abilities of the Upper-Rank demons were shown in the Infinity Castle Arc, where the Hashira and other members of the Corps came face to face with the Upper rank.
Leader
Muzan Kibutsuji

Muzan is a highly intelligent demon. This is why he can easily manipulate people and create a strategy to defeat the members of the Demon Slayer Corps. He can change his appearance, so nobody knows what he looks like. He was only identified by his smell which only Tanjiro could do. Muzan is a narcissist, so he cannot stand losing and having another strong person in the same room as him. He is intimidating and ruthless. This was shown when he killed all Lower Six of the demons when he no longer saw them fit to serve him.
Upper Ranks
Upper Moon Rank 1: Kokushibo

In his death, he was a former demon slayer who made a deal with Muzan to make him a demon. He is extremely loyal to Muzan, and as a result, all his actions are to fulfill the wishes of his master. He envies his twin brother for excelling in a lot of things. Being the eldest of the two brothers, Kokushibo has everything, wealth, education, and all the inheritance in their household. Yoriichi, however, had proven to be the better swordsman.
Upper Moon Rank 2: Doma

Belonging in the ranks of upper demons, Doma does not have the appearance that will scare people. He is instead friendly, calm, and carefree. He is also approachable. He has a cult where he eats his followers believing that they are given the best chance in life by living within his body. He was the one responsible for the death of the Kocho sisters. While he failed to eat Kanae, he was successful with Shinobu.
Upper Moon Rank 3: Akaza

Akaza surely is one of the strongest demons. He wanted to argue and raise his ranks, but he could not win with any of those with higher ranks than him. He is a battle-crazed demon, and he has high respect for those who are stronger than him. When he faced Giyu, he demanded his name to which Giyu responded that he did not deserve to know his name. Akaza, as a human, was forced to steal because of his ailing father. He was beaten until he was numb, but he endured it.
Upper Moon Rank 4: Hantengu

Hantengu looks like a frail old man. He is so frail that it seems he cannot commit murder. Since he was a human, he has always believed he could not commit a crime. It was still the same as when he was already a demon. He still believes that he is innocent when he eats humans and kills them.
Upper Moon Rank 4: Nakime

She is the one used to summon the demons. When Hantengu died she replaced him and became the Upper Moon 4. She became a demon by killing people before she plays her biwa. The first murder she committed was that of her husband. Her husband sold her dress and used it to gamble so she killed him with a hammer. The sensation she felt when killing her husband made her fingers tremble that the sound she created was so beautiful so she continued killing before she performs. Muzan was one of the people she is supposed to kill before her performance. Muzan was impressed and offered to make her a demon.
Upper Moon Rank 5: Gyokko

Gyokko’s real form was something that resembles a merman. His parents died when he was a young boy. They were out fishing and when they died, their bodies were severely deformed Gyokko took a fascination with it and he started collecting carcasses of fish. He killed a boy and stuffed his body in a jar. The parents of the boy killed him with a two-pronged fork. While dying, Muzan happens to pass by and made him into a demon.
Upper Moon Rank 6: Gyutaro and Daki

Gyutaro was the brother of Daki. He used to be her collector in the Red Light District. His mother had attempted to kill him since he was born but to no avail. He was extremely poor and the birth of his sister gave him extreme joy. He killed the samurai and the manager who wants him dead and while walking around the district carrying the almost dead body of his sister, they met Doma who gave them his blood and made them demons. Daki was a courtesan in her human life and was tied and burned to death by a samurai she poked in the eye. When they were saved by Doma, she and her brother fed on the best geisha and oiran in the district, and in the end, they were made Upper-Rank demons by Muzan. She was known as Ume when she was human and she has an extreme love for her brother. They both share rank 6.
Lower Ranks
Lower Moon Rank 1: Enmu

Enmu always had problems with differentiating between dreams and reality. He was practicing hypnosis and would tell patients that they are getting better even if they are not. This had caused so many problems. When he was about to be devoured by Muzan he praised him and Muzan decided to spare his life and make him a demon.
Lower Moon Rank 2: Rokuro

Little is known about Rokuro aside from him asking for more blood from Muzan, saying that he will be stronger if he had more blood from him. He was the sixth to be killed on screen.
Lower Moon Rank 3: Wakuraba

Wakuraba was the one who ran out of the Infinity Castle when he discovered that Muzan would not spare any of them. He ran as fast as he could but his head was still chopped off by Muzan and Wakuraba does not know how he died.
Lower Moon Rank 4: Mukago

He is the most cowardly of all the lower ranks. He thought to himself that whenever he meets a Hashira his first thought is to run away from them. Muzan killed him when Muzan had read his thoughts.
Lower Moon Rank 5: Rui

Rui was born from an affluent family but with a frail body. Muzan offered to make him a demon and give him a strong body. He killed his parents and then created a family that will love him unconditionally. This means the new member of the family will continuously protect him at all costs. Rui was defeated and killed by Giyu Tomioka.
Lower Moon Rank 6: Kamanue

Kamanue was the lower rank 6 who replaced Kyogai. He was killed by Muzan for talking back to him. While he cannot talk back directly to Muzan, he was thinking that Muzan cannot just talk like that to them and so Muzan killed him.
Lower Moon Rank 6: Kyogai

Kyogai was a former member of the twelve Kizuki. He lost his ability to eat humans and stopped evolving. Muzan then removed his mark. Kyogai went on a journey to find how he can be restored as part of the Kizuki again.
Other Demons
Spider Demon Family

They are the demon family that Rui enslaved and make his new family.
Tamayo

Tamayo is an extremely talented doctor. While she is a demon and needs blood to consume, she makes sure that she buys the blood she drinks. She is highly intelligent and has knowledge of Demons and Demon Transformation. In the early years, when Muzan was still her master, she stopped avenging her family. When she saw what Yoriichi, a mere human, can do, she decided she wants to continue with her plan to avenge her family.
Yahaba and Susamaru

They aspired to join the Kizuki. They attack the Kamado siblings while they were visiting Tamayo to ask about healing Nezuko. They were the ones summoned by Muzan when he met Tanjiro in Asakusa. Muzan got irritated seeing a demon slayer wearing Hanfuda earrings so he summoned these two to hunt them down.
Yushiro

Yushiro is the assistant of Tamayo and is greatly in love with her. He vowed to protect her at all cost. After the war with Muzan, Yushiro is the sole survivor of the demon race. He became a famous painter and draw a total of 812 paintings of Tamayo.
Kamado Family
Tanjuro Kamado

The father of Tanjiro kamado appeared in his flashback while he battles the Lower Rank demon, Rui.
Takeo Kamado

The second oldest Kamado son.
Kie Kamado

The mother of Tanjiro.
Sumiyoshi

The ancestor of Tanjiro received the Hanafuda earrings from Yoriichi, the first Breath of Sun user.
Swordsmith Village
Hotaru Haganezuka

He is the man who made the sword of Tanjiro and another demon slayer.
Tecchin Tecchikawahara

He is the chief of the village.
Tetsuido

He was the one who is responsible for the sword of Muichiro Tokito.
Kotetsu

The boy who holds the key to a doll that they use for training their swordsmanship as the doll is better than a human.
Senjuro Renguku

His parents are Shinjuro Renguku and Ruka Rengoku. He is the younger brother of the Flame Hashira, Kyojuro Renguku. Senjuro is supposed to succeed Kyojuro but he is too weak and does not have the ability to do so. He helps people in different ways instead. He was the one who restored the Generational Flame Hashira book that his father torn to shreds.