Tsukihi Araragi is the youngest member of the Araragi family, the younger sister of Koyomi and Karen Araragi. She forms one half of the "Fire Sisters" duo alongside Karen at Tsuganoki Second Middle School, though she admits to lacking Karen's innate drive and views justice more as a hobby than a calling. Calm and traditionally feminine in demeanor, she often wears brightly colored yukatas or kimonos at home. This demeanor conceals a volatile temper described as hysterical by Koyomi; she can become impulsively aggressive when angered, sometimes resorting to dangerous objects while smiling. Despite this intensity, she is popular among peers and maintains a chaste relationship with her boyfriend, Rousokuzawa.

Tsukihi is not biologically human but an immortal oddity known as the "Dying Bird" or "Shide no Tori," a phoenix-like entity that reincarnated by replacing an unborn child. This resulted from Seishirou Shishirui's vampiric influence affecting her mother during pregnancy. Her true nature grants rapid regeneration; life-threatening injuries heal instantly, while minor wounds recover at near-human speeds. This ability also accelerates her hair growth, leading to frequent hairstyle changes from a Dutch bob in her initial appearances to ankle-length braids later. She remains unaware of her supernatural identity and dismisses oddities.

Her story arc in *Nisemonogatari* ("Tsukihi Phoenix") unveils her immortality. When oddity specialists Yozuru Kagenui and Yotsugi Ononoki attack, Yotsugi obliterates Tsukihi's upper body with her "Unlimited Rulebook." Tsukihi regenerates immediately, confirming her nature. Shinobu Oshino later explains she is a "fake" human whose existence will end after a normal human lifespan, prompting the bird to reincarnate anew. After recovering, Tsukihi chastises Koyomi for stealing her first kiss during a previous incident, though they reaffirm their sibling bond.

Continuing to live as a human in later arcs, she and Karen provide shelter for Tsubasa Hanekawa during *Tsubasa Tiger*. She appears with unusually long hair shortly after *Nisemonogatari*, attributed to her accelerated hair growth. She interacts sporadically with Koyomi, advising him on love and expressing concern over his self-sacrificial tendencies. Her friendship with Nadeko Sengoku, nicknamed "Rara," dates back to elementary school, though Nadeko's later actions strain their relationship.

Post-graduation events in *Musubimonogatari* reveal Tsukihi studied abroad five years later but dropped out. She returns to Japan aspiring to join a dance school and perform at a casino. Her whereabouts during Koyomi's wedding are unknown, as she does not attend. Her catchphrase, "I'm platinum mad!" (often mistranslated as "dagnabbit mad" in English releases), recurs, and she sings the theme "Platinum Disco" during her arc.