Koyomi is a high school student that was formerly a vampire and still maintains his vampire power, strength, and regenerative abilities. It is later revealed that he maintains these abilities because of the bond he shares with his vampire sire Shinobu, who appears to be an eight-year-old girl but is more than 500 years old. Because of this fact Koyomi is often concerned about having an extremely long lifespan like Shinobu and, therefore, outliving all of his friends and family. In the first installment of the Monogatari series, he’s consistently running into girls who have been possessed or affected by other supernatural oddities called apparitions and feels compelled to help them.
And because of this princely need to save the five girls he encounters Koyomi becomes their unintentional harem king, though he admits to being in love with his girlfriend, Hitagi. He cures the girls of their strange supernatural maladies with help from Meme Oshino, his middle-aged onmyoji (magic specialist) friend who is well versed in the supernatural. Most of the girls are cured by confronting a deep-seeded childhood trauma or confessing a unexpressed emotion. Koyomi often gets into physical altercations with the supernatural creatures that are possessing the girls and uses his supernatural strength and healing ability to hold his own.
The Monogatari series is full of running jokes, wordplay, and veiled references to other anime series most of which come from Koyomi. Because of this fact his interactions with the other characters in the series are often extremely comical especially those with Hitagi and Mayoi. All of the girls are incredibly flirtatious with Koyomi, and his responses to their advances are hysterical. And like any good harem king Koyomi is often extremely sexually perverted with all of the girls in the series to include his two younger sisters. Also, Hitagi mercilessly teases him and threatens him with her deadly arsenal of school supplies if she feels insulted by him.
In Nisemonogatari, the five girls he encounters are all cured of their supernatural problems, however, now his two younger sisters are facing similar oddities. His sisters are completely unaware of his vampirism, and they do not know about the other apparitions that he's faced. They do know that when Nadeko was cursed Koyomi helped her though they don't know the details of the situation. When his sister Karen is poisoned by a supernatural bee, he takes half of her pain away and tracks down the person responsible for her poisoning. Later we learned that his youngest sister Tsukihi is an immortal Phoenix in human form. When a female onmyoji and her shikigami come to town, they tried to kill Tsukihi. As a result, Koyomi risked his life fighting against them and kept his little sister safe from harm.
Karen is the oldest of Koyomi’s younger sisters. She takes martial arts classes and is always engaging in physical altercations and competitions with Koyomi much to his chagrin. Despite being three years younger than Koyomi, Karen is taller than he is, and she believes herself to be stronger. Because of her physical prowess and athleticism she naturally idolizes Koyomi ’s underclassmen Suruga Kanbaru, who is a basketball star. It is because of her desire to meet Suruga that she agrees to participate in a challenge of Koyomi’s choosing. Which resulted in the ever so controversial toothbrush scene( a scene where Koyomi brushes Karen's teeth, and both seem to experience sexual pleasure from the action).
It is suggested that Karen is sexually and romantically interested in her brother. She is hot-tempered and often refuses help from anybody and acts without thinking first. She and her younger sister believe themselves to be modern day superheroes known as the fire sisters, and they stand up for what is right and help others, much like their older brother. It is because of this act that Karen confronts Deishuu Kaiki, a con man who has been selling middle schoolers charms and curses all in the name of making a profit. We also learn that it was one of his curses that was placed on Nadeko Sengoku and it nearly killed her. However, when she goes to confront him alone, she is stung by a supernatural bee and is in pain and bedridden for three days. The pain was so intense that Koyomi had to take some of it from Karen to prevent her life from being in danger.
Tsukihi (which means fire of the moon) is Koyomi‘s youngest sister. She is depicted as being a lot more sweet and feminine than her older sister Karen. However, while Tsukihi doesn't have Karen's physical prowess and strength, Koyomi is actually more afraid of her due to her erratic behavior and short temper. She usually wear kimonos and yukatas and is constantly changing her hairstyle due to her fickle nature. She's not as interested in preserving justice or helping those in need as her brother and sister, and often only goes along with being a fire sister out of boredom. She does not care for outdoor activities and would prefer to stay home and monitor the antics of her older siblings. We are told that she has a boyfriend named Rosokuzawa and that she is in love.
We learn that she's, in fact, a reincarnated Phoenix, and has been since before she was born. The Phoenix’s spirit blends with the unborn child until the two are one and the same. She is immortal and can only die of old age. Yozuru Kagenui and her shikigami Yotsugi Ononoki threaten to exterminate Tsukihi, so Koyomi and Shinobu fight them in order to keep Tsukihi safe. Koyomi pleads with Yozuru to leave his sister alone because she never engages in any evil activities, and he will be there with her to ensure that she stays out of trouble. Yozuru upon hearing this decides to abandon her mission to eliminate Tsukihi. Koyomi never tells Tsukihi that she is an immortal Phoenix.
Hitagi is a beautiful, yet anti-social girl that is in the same class as Koyomi. Koyomi discovers that Hitagi has a supernatural ailment, so he immediately takes her to see Oshino in order to solve her problem. There we find out that Hitagi encountered a giant crab apparition that took away all of her physical weight, leaving her frail and weightless. Hitagi is quick-witted, sarcastic, and has a sharp matter-of-fact way of speaking. She's consistently getting into hilarious, verbal sparring matches with Koyomi, which seems to be how she shows her affection towards him. In addition, she also carries a large amount of school supplies on her at all times that she uses as deadly weapons, to include staplers, sharpened pencils, and Exacto knives. She often threatens Koyomi with these weapons if he's offended her or caused her to be jealous.
She regains her physical weight when she confronts the crab apparition and faces the traumas in her past such as losing her mother to a cult and having almost been raped by the cult leader. In addition to regaining her weight, her personality in some respects begins to soften, and she admits to Koyomi that she's in love with him, and they begin dating. In Nisemonogatari, Hitagi has to confront Deishuu Kaiki, the con man that failed to help her cure her supernatural illness and whom she blames for her parents splitting up as a result. In the end, she's able to let go of her grudge toward Deishuu Kaiki and focus on her relationship with Koyomi.
Mayoi is the ghost of an 11-year-old 5th-grade elementary student, who died 11 years ago when she was a hit by a car while on her way to see her mother. Her parents divorced a while back with her father maintaining custody of her, so she had not seen her mother in a very long time. Because she never reached her destination, Mayoi has become a snail apparition that causes others to lose their way. When Koyomi first encounters her, he is unaware that she's a spirit until Hitagi confesses to him that she can not see the child that he's been talking to all day. Koyomi helps Mayoi find her way to the vacant lot where her mother once lived and in doing so frees her from being a snail apparition.
Though she no longer causes others to lose their way and she stays a wandering spirit. Mayoi is constantly misspelling things and mispronouncing Koyomi’s family name. Koyomi constantly teases and sexually harasses Mayoi, which leads her to believe that he is secretly a lolicon (someone into young girls or those that look young). Koyomi states that he's happiest when he is speaking to Mayoi, and expresses concern about her passing on to heaven without saying goodbye to him first. Mayoi is consistently offering Koyomi advice and reminding him that she has child’s body but an adult mind because technically she's 22 years old.
Suruga is a basketball star and Koyomi’s underclassmen. She attended the same middle school is Hitagi, and they had a close friendship at the time. Suruga, like Koyomi, discovered Hitagi’s secret illness, and Hitagi threatened her, which ended their friendship. Suruga admits to Koyomi that she is a lesbian that has feelings for Hitagi (though she frequently flirts with Koyomi and jokes about becoming his mistress one day). She is also a lolicon, and she is completely obsessed with boy love novels and short stories, to the point of them being all over her room. Suruga lives with her grandparents because both of her parents are deceased.
We learned that she inherited what she believed to be a monkey's paw in elementary school upon her parent’s death. In middle school, she was quiet and reserved which led to her being bullied quite a bit. She read that she could use the monkey's paw to make wishes. She tells Koyomi that she made a wish for her to win a race against her classmates, figuring that this would make her popular. The day before the race the other students she was going to race against were hospitalized. Suruga shortly thereafter reads a horror short story called The Monkey's Paw by W.W. Jacobs where the monkey’s paw grants wishes, but only at a horrible price. Figuring this was the case with her monkey's paw she used her second wish to undo the first wish for her to be fast enough to win the race. But she still didn't feel safe enough to try to race again so rather than participating in track, she turned her sights to basketball instead. When she sees that Koyomi and Hitagi are getting closer, she regrets not helping cure Hitagi’s supernatural melody when she had the chance. So she decides to use her last wish to help cure her. However, rather than granting the wish the monkey's paw bonds with her arm, and takes on a devilish life of its own. For example, it completely on its own accord, tracked down Koyomi and beat him within an inch of his life.
Oshino tells her that it was not a monkey's paw at all, but an oddity known as a rainy devil that had possessed her arm. And that the reason these things happened was because of her subconscious desire for them to happen. The kids in the middle school were hospitalized because the rainy devil arm attacked them the night before, and Suruga attacked Koyomi because she is extremely jealous of his relationship with Hitagi. When Suruga was forced to confront this fact, she flew into a rage and attacked Koyomi once again, only coming under control when Hitagi appears, and talks her down. Suruga confesses her love to Hitagi, and Hitagi says that while she does not reciprocate her feelings she will always be by her side, and love her as a friend. This undoes the curse of the rainy devil arm, and Oshino says she should be completely rid of the arm by the time she turns 20 years old.
Nadeko is a childhood friend of Koyomi’s sister Tsukihi, and would often come to the house and play with both of them. Nadeko is an extremely shy girl, who lacks self-confidence and often look at the ground when speaking to people. In the first installment of Monogatari, Nadeko shows Koyomi that she has a snake like bruise coiled around her entire body. Nadeko tells Koyomi that she believes that one of her classmates put a snake curse on her, and that she was going to the snake temple and killing white snakes in an effort to remove the curse. However, it only made the bruises hurt more.
When Koyomi asks Oshino about the snake he tells him that they have to perform a cleansing ritual immediately at the snake temple or Nadeko is going to die. Koyomi must take care to thoroughly exorcise the snake curse or else it will go back to the person who cursed Nadeko. and curse them in her place. Koyomi with the help of Suruga, was able to successfully exorcise and fight off the snake curse however, in the process of performing the ritual Koyomi learns that Nadeko has been cursed not once but twice. Koyomi realizes that the boy who Nadeko rejected also put a snake curse on her. And though he tries to fight it off, he was unable to prevent it from going back to the boy and cursing him as well.
Nadeko quite obviously has a crush on Koyomi, and her feelings for him is the reason that she rejected the boy in the first place. In Nisemonogatari, we learn that Deishuu Kaiki is responsible for selling both of Nadeko’s classmates the snake curse. While she is a major character in the first installment of Monogatari her involvement in Nisemonogatari is limited to playing a highly sexual game of Twister with Koyomi when he comes by to visit.
Tsubasa is Koyomi’s classmate, and she is also the class representative. We find out that Koyomi has had a crush on Tsubasa for a very long time. We also discover that Tsubasa was his first encounter with another oddity. Tsubasa has been possessed by a bakeneko (cat spirit) and the spirit takes over anytime Tsubasa is plagued by stress. Both of Tsubasa’s biological parents are deceased, so she lives with her stepfather and his new wife who are neglectful and mistreat Tsubasa. It is because of her family stress that the bakeneko first appears during Golden Week (Apr. 29-May 5th a week full of Japanese holidays).
During Golden Week Koyomi sees that Tsubasa has a bandage on her bruised cheek, and she explains that she got the injury when her “father” slapped her. When Koyomi says, she should get some help she defends her father's actions, and begs Koyomi not to tell anyone. Shortly thereafter, she finds a white cat with no tail dead on the side of the road and gets Koyomi to help her bury it. At that moment she gets possessed by the cat demon, and that evening she drains the life force from numerous town’s people including her parents. When the cat demon has control, it's known as her Black Hanekawa state. She loses control once again when Koyomi starts dating Hitagi, and she has to confront her feelings of jealousy.
On both occasions, Koyomi had to fight against her and was only able to subdue the cat demon with help from Shinobu. Oshino tells Koyomi that cat demons are usually low-level spirits but because it possessed Tsubasa this spirit has power and strength that rivals that of a vampire. In Nisemonogatari, Tsubasa’s cat demon spirit is under control and she has taken the fire sisters under her wing and is acting as their mentor.
Shinobu is a powerful vampire who is over 500 years old, and was originally named Kiss-shot Acerola-orion Heart-under-blade. She originally appeared in adult form, but loses most of her power and is reduced to the appearance of an eight-year-old girl after attacking Koyomi and turning him into a vampire. For a while, she lives with Oshino, who renames her, and she sustains herself off of Koyomi’s blood. After she helps Koyomi defeat Black Hanekawa, she begins permanently living in his shadow.
In the first installment of the Monogatari series she does not speak and is very somber. However after "reconciling" with Koyomi in Nisemonogatari, her personality becomes much more bright and bubbly, and she becomes extremely talkative. She becomes a vital reference for oddities and the supernatural much in the same way as Oshino was in the first installment. For example, she is the one who tells Koyomi how to remove the bee poison from Karen in order to save her life.
And when he discovers that his sister Tsukihi is really an immortal Phoenix, it is Shinobu that explains what a Phoenix is and how his sister came to be one. She's addicted to donuts and frequently makes Koyomi buy them for her. She shares a deep bond with Koyomi and can feel his pain, and his emotions and all of his strength and regenerative abilities are a direct result of this connection. She joked that because of their long lifespan that they will be together forever. Koyomi responded that he would gladly be with her for eternity and died with her if need be.
Yozuru Kagenui and Yotsugi Ononoki
Yozuru is an onmyōji and attended the same university as Oshino and Deishuu. She always wears high heels and never for whatever reason steps on the ground but rather is always balancing on fences, sign posts, etc. For a human, she's amazingly strong, and she was able to beat Koyomi to a bloody pulp during their fight over Koyomi's sister Tsukihi. Tsukihi is an immortal Phoenix that Yozuru wants to kill. She specializes in hunting and killing immortal oddities. She has a soft spot for Koyomi, which is why she inevitably relinquishes her desire to kill his sister. But she states that if he or she steps out of line and turns to evil she will not hesitate to be back and eliminate the both of them.
Yotsugi is her shikigami and is in possession of the Unlimited Rulebook, an ability that allows her to shapeshift her finger into a number of weapons. She uses this ability on Koyomi’s sister Tsukihi and completely destroys the upper half of her body. However, due to the regenerative powers of the Phoenix she recovers within moments.
Deishuu Kaiki is a con man that sells middle school students charms and curses. When Koyomi’s sister Karen goes to confront Deishuu Kaiki about his shady business dealings by herself she ends up being stung by a supernatural bee and is bedridden for three days as a result. Even though he attended the same university as Oshino and Yozuru, he does not believe in the existence of oddities and rather he uses his extensive knowledge of them to trick people out of their money. We discover that he was the first of five con men to trick Hitagi ’s family out of their money when she lost her weight to the crab and that she holds him responsible for her family breaking up.
When Hitagi confronts him about the role that he played in her family breaking up she is finally able to let go of her hatred and move on. We also learned that it was Deishuu that sold information about snake curses to the two middle school students that eventually cursed Nadeko. He loves fake things because he believes fake things to be realer than real things. Which, may be how he can reconcile the paradox that he is a fake oddities specialist that has a wealth of real knowledge about real oddities. He's not above playing both sides when money is concerned. He answered Koyomi’s questions about Yozuru and told him that she’s an onmyoji, but he also sold the information that Tsukihi is a Phoenix to Yozuru.