Tsubasa is Koyomi's classmate, and seems to have known Senjogahara since middle school. She seem to be very intelligent and is described by Koyomi as the "the class president of all class presidents."
About a year before the beginning of the series, Tsubasa revealed to Koyomi that her parents abused her at home, and she even put some gauze on her face from the most recent episode when her father hit her. She begged Koyomi not to tell anyone, saying that it was her fault for not doing enough good work. She kept blaming herself for her parents' abuse, but continued keeping up a positive exterior.
One day, she tried to help a cat lying on the side of a street. Suddenly she got strange headaches until cat ears emerged from her head, her hair turned white, and she climbed a tree. Oshino told Koyomi that she had the characteristics of a cat now, and that the stress of her home and school life placed such a heavy burden on her that her subconscious mind took over in the form of a white cat. As a cat, Tsubasa prowled the rooftops and stalked her prey, the major source of her stress: her parents. Once she leaped into action, she drained them of all their energy, then set her sights on Koyomi. Koyomi thought that she would do the same to him, so he led her into a trap where Shinobu emerged out of nowhere and sucked out Tsubasa's energy with her vampiric fangs until the cat receded and Tsubasa returned to normal.
Tsubasa is a character from the Monogatari franchise and has made appearances in the light novel series written by Nishio Ishin, featuring illustrations by Vofan, and published by Kodansha. She has also made appearances in the anime series based off of these light novels: Bakemonogatari and Nisemonogatari, both of which are produced by Shaft and published by Aniplex . Her original design is credited to the writer and illustrator of the light novel series while her anime appearance is credited to Akio Watanabe. She makes her first appearance in Bakemonogatari #1.
In the present, Tsubasa is starting to get strange headaches again, and Araragi wonders if the cat is returning. Indeed, in the latest arc, she turned back into the white-haired cat right in front of Araragi and Oshino. Unlike last time, Koyomi can't find Shinobu to take some of Tsubasa's energy away, so Tsubasa ends up following him around in cat form while he calls the other girls he has met over the series to find Shinobu for him. The cat suggests that Tsubasa's stress this time comes from being in love with Koyomi but being unable to express that love. However, if he confesses his love for Tsubasa, the cat might go away. Eventually, Shinobu appears and is able to revert Tsubasa back to her normal form.