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 hair style and is short-tempered to the point that Koyomi describes her as having hysteria.
She is actually the reincarnation of a phoenix and has been since before she was even born. The phoenix entered the baby's body when it was still in the womb and by now both beings' minds have become one. She is immortal and the only thing that end her life is a natural death from old age. It was hinted that Kaiki was the one who sent Kagenui and Ononoki to go after Tsukihi. Tsukihi and Karen are nicknamed Tsuga no Ki Nichū no Fire Sisters (栂の木二中のファイヤーシスターズ). It was revealed that the reason she and Karen found boyfriends was so that Koyomi would become jealous. They found boyfriends in Koyomi's image with Tsukihi's boyfriend being taller than her. She and Karen also thought that Koyomi's girlfriend was Tsubasa. Koyomi, Karen and Tsukihi's parents were revealed to be police officers.