Araragi, Tsukihi

Tsukihi, the main character of Tsukihi Phoenix, is one of Koyomi's sisters. She's the youngest of Araragi family. Unlike her sister, Karen, she is always doing indoor activities. Contrary to her personality, she constantly changes her hairstyle and is short-tempered to the point that Koyomi describes her as having hysteria.

Kagenui believed the Tsukihi was a phoenix, but she is actually the reincarnation of a supernatural cuckoo called the "Dying Bird." The Dying Bird reincarnates by entering the body of a child still in the womb. She is immortal and the only thing that will end her life is a natural death from old age. It's hinted that Kaiki was the one who sent Kagenui and Ononoki to go after Tsukihi. Tsukihi and Karen are nicknamed Tsuganoki 2-chuu no Fire Sisters (栂の木二中のファイヤーシスターズ).

(Source: Wikipedia)