Tsukihi Araragi, the youngest sibling in the Araragi family, is the biological sister of Koyomi and Karen Araragi. She attends Tsuganoki Second Junior High School and partners with Karen as one half of the "Fire Sisters" justice duo. Unbeknownst to her for most of her life, Tsukihi is not human but an offspring of the Shide no Tori—a supernatural entity called the "Dying Bird" or "fake phoenix." This entity implanted itself in the Araragi matriarch's womb to reincarnate, granting Tsukihi immortality and rapid regeneration. Her appearance matches a young teenager, featuring black hair, dark gray eyes, and the characteristic Araragi ahoge (cowlick). Her regenerative abilities cause accelerated hair growth, leading to frequent hairstyle changes. Tsukihi almost exclusively wears brightly colored yukatas and kimonos of varying lengths, only rarely seen in her school uniform or modern casual clothes. Personality-wise, Tsukihi displays a calm, polite, and feminine facade, yet conceals a volatile temper Koyomi labels "hysterical." Unlike the more active Karen, she prefers indoor pursuits and admits to lacking her sister's drive for justice, though she shares the family's ideological commitment to it. Remarkably airheaded, she rejects belief in the supernatural despite being an oddity herself. Her explosive outbursts feature the catchphrase, "I'm platinum mad!" She maintains a chaste relationship with her boyfriend, Rousokuzawa, and shares an intensely close bond with Karen, whom Koyomi jokingly calls her "yuri sister." Though she frequently bickers with Koyomi, she deeply cares for him, offering advice on topics like love, while showing less overt deference to him than Karen does. Her background stems from the influence of the vampire Seishirou Shishirui's aura on the town. In elementary school, she befriended Nadeko Sengoku (who nicknamed her "Rara-chan") and witnessed Sodachi Oikura's brief stay with the Araragis, though she later claims no memory of Sodachi. As the Dying Bird, she possesses true immortality: fatal injuries like the obliteration of her upper body by Yotsugi Ononoki trigger instant regeneration. However, her human lifespan is finite; upon natural death, the entity will reincarnate anew. This regeneration also explains her constant hairstyle shifts and high metabolic rate, requiring frequent baths. Key developmental moments include "Tsukihi Phoenix," where her true nature is exposed. After Yotsugi's attack and instant recovery, Shinobu later explains her origin to Koyomi. Initially shocked when Koyomi steals her first kiss to "confirm their sibling bond," she reaffirms their familial ties, strengthening his resolve to protect her. In "Tsukihi Undo," she skips school, discovers Yotsugi is animate, and tests her vitality by trying to set her ablaze. After Yotsugi fabricates a story about being a magical girl, Tsukihi forgets the incident by the next day. Later, in "Sodachi Lost," she and Karen deny remembering Sodachi's past stay. Her absence from Koyomi's wedding in "Hitagi Honeymoon" hints at unresolved familial dynamics. Trivia notes her name's meaning ("moon fire" or "Monday-Tuesday"), the kanji correlation with Koyomi ("calendar"), and her role narrating anime previews alongside Karen.