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.
From Wikia, continue to read it there~ :D