Hitagi Senjougahara (戦場ヶ原 ひたぎ, Senjōgahara Hitagi) is a third-year student of Naoetsu Private High School. She is considered to be a weak-looking girl with an "incurable disease" and is distant towards others. In an accidental encounter with Koyomi Araragi, her closely kept secret was exposed, and this triggers a life-changing experience. In the course of the series she receives the nickname Gahara.
Hitagi is a girl with long purple (sometimes portrayed as dark brown) hair and blue eyes. She has a glamorous figure, although she is also considered as a "frail" girl due to her lack of participation in school activities. She wears the Naoetsu Private High School uniform the usual way while wearing black thigh-highs and can be seen wearing dresses or similar clothing that cover up to her knees.
In later events, she has the sleeves of her uniform rolled up and pinned in place, turning her uniform into a sleeveless blouse.
Hitagi possesses a sharp tongue and can be blunt while still maintaining a straight face when speaking to others. This trait of hers makes interacting with her a challenging task, if not impossible, something that Araragi discovers first-hand. In spite of this, she is sincere in everything that she says.
She calls herself a tsundere and probably has some idea about otaku culture, as she once mentioned how "moe" Meme's name was to Araragi.
For the three years of her high school, Hitagi is in the same class as Koyomi, although she has rarely spoke with anyone at that time. Likewise, Tsubasa Hanekawa knows little of Hitagi's current state, only judging her from what she knew back in middle school.
According to Tsubasa, Hitagi was an overachiever in middle school, friendly towards others and excelling in both academics and sports as a varsity track and field athlete. She was also wealthy, as her father is famous in a certain foreign corporation and she lives in a mansion; despite this, she retains a sense of humility. Her seemingly perfect personality grants her the admiration of her classmates and teachers.
When she was in the first year of high school, she encountered a mysterious crab, after which she became weightless. Ever since, she has avoided contact with everyone else, and she threatens everyone who discovers her secret.
Hitagi reveals that she was a girl who had a close brush with death when a disease threatened her life, barely surviving the ordeal through an extensive surgery. What happened to her caused her mother to seek refuge to a cult, which eventually initiated the deterioration of the Senjougahara household. Hitagi's mother sold the family's riches to the cult and her involvement with the cult almost caused Senjougahara to be sexually assaulted by one of the cult's high-ranked members. Eventually, Hitagi's mother left the family, leaving behind a large debt which Hitagi's father is currently trying to pay back.
In an accident involving a misplaced banana peel, Hitagi falls down the flight of stairs that lead to the upper floors of Naoetsu Private High School and ends up in the arms of Koyomi Araragi, her classmate. Later that day, she confronts Araragi about how he found out her secret and threatens to hurt him unless he promises to keep quiet about his discovery. Koyomi, in spite of this, declares his intention to help, showing the completely healed wound where Hitagi implanted a staple into his cheek moments earlier. Shocked by Araragi's immediate recovery, Hitagi decides to join him on his trip to the person who might be able to turn her back to normal: an eccentric man named Meme Oshino.
She learns from Meme that she was inflicted by an oddity called a "heavy stone crab" (or "god"), who took away her weight in a past encounter with it. Although reluctant to cooperate with Meme, who reminded her of the scammers who once visited her with the same promises of turning back to normal, she decides to go with Meme's methods, even willing to pay 10,000 yen for his services.
After changing into a set of clean clothes as part of the ceremony planned by Meme, Hitagi and Araragi return to Meme's place. Hitagi is then put into a situation where she would recall the memories of her life. Afterwards, the heavy stone crab manifested in front of her, visible only to Hitagi herself. Although the unpredictable behavior of the entity forced Meme to intervene, Hitagi manages to plead to the heavy stone crab to lift his influence from her. The heavy stone crab grants her wish and departs, leaving behind Hitagi with the burdens of her childhood on her shoulders. She begins to cry while Meme explains her resolution.
Hitagi, after the ordeal with the heavy stone crab, thanks Araragi and he becomes her first friend after a long time.
Hitagi meets Araragi by chance in the town's park dressed with a set of clothes she just bought. She and Araragi talk in the park about several things, including how she would repay him for his help in turning her back to normal, and she considers unusual methods to return the favor, much to Araragi's shock.
Soon, Araragi's attention would be shifted towards a young girl who seems lost. Following Koyomi's advice, they help the girl, named Mayoi Hachikuji, on her way back home, but they soon find themselves unable to reach the address. They later decide to consult Meme about Mayoi, and they discover that she is a lost cow (a play on the word "snail" (蝸牛, katatsumuri) which has the character for "cow" (牛, ushi)).
On her return trip to the park where Araragi is, she finally confesses that she had been pretending to have been sensing Mayoi's presence and that Araragi is the only one inflicted by the oddity (i.e. Mayoi herself). According to Hitagi, Mayoi was already dead years ago, and people who are victims of the lost cow often have problems at home, and this causes Araragi (and Tsubasa, who also has problems of her own at home) to be able to see and interact with Mayoi. Afterwards, Hitagi decides to help out with Mayoi's problem, showing an alternate route that is supposed to confuse the oddity by introducing roads that Mayoi has not seen before.
Hitagi, Araragi and Mayoi soon reach the remnants of the former Hachikuji residence, now simply an empty lot, and the two witness Mayoi's fulfillment of her final objective before moving on. After the accomplishment of their mission with Mayoi, she admits that she loves Koyomi, and she subsequently enters into a relationship with him.
To prepare for the school's midterm examinations, Senjougahara prepares a group study session with Araragi. In one of those sessions, she prepares the money she owed Meme after curing her of her unusual condition. However, Araragi forgets to take the money with him, so Hitagi goes to catch up with him. She would later see Araragi in a poor state, badly wounded by an attack that he tried to explain as "a bike accident gone horribly wrong" and helps him recover in her own unusual way.
Hitagi would soon hear about Araragi facing her middle school friend Suruga Kanbaru from Meme himself and, seemingly as Meme planned, intervenes the Rainy Devil's relentless attacks against Araragi. After scolding Araragi for dealing with things without her knowing about it, she confronts Suruga, telling her that she will not forgive anyone who would kill Araragi before reconciling with her friend. After the Rainy Devil left Suruga behind, the two become friends once again.
Senjougahara decides to give Kanbaru some time together with Araragi so that the two can be friends after the events involving the Rainy Devil. She even allows Kanbaru to choose a necessary punishment for him if ever Araragi gives her trouble, much to Koyomi's worry.
Hitagi, under the excuse of "protecting", captures Koyomi and detains him inside one of the many classrooms inside the abandoned cram school building where Meme stayed for some time. During this time, Hitagi would provide food and drink to Koyomi, while he is chained up to the wall. She later allows Koyomi to get away from his restraints after she relays a message concerning his sister.
After the events in Tsukihi Phoenix, she overcame all of her trauma, and finally became a completely common cheerful girl. She starts referring to Araragi with a cute nickname, chuckling at small things and sending e-mails full of Emojis, although her sharp tongue is still there, but toned down. She seems to also have a father complex, getting excited when she uses her father's blanket, so much so that she can't fall asleep.
Koyomi says that he loves Hitagi the most. She seems to not be on very good terms with Karen and Tsukihi. So far in the latest novels, she is either just mentioned or just plays a small part.
In Nadeko Medusa, Nadeko says that if they were to meet in different circumstances, they would have been friends but Hitagi says that she hates cute brats like Nadeko.