Perishing in a car crash at age sixteen, Itsuki Yamagami returned as a Shikabane Hime fueled by regret over her lost teenage years. She contracted with Takamasa Sougi, a Kōgon Sect monk, mistakenly believing his power could hasten her path to heaven. Despite Takamasa's initial ignorance of Shikabane Himes, their partnership formed.

Itsuki manifests as a teenage girl with red hair often adorned by a large pink bow, brown eyes, and a 154 cm stature. This typical high school student appearance aids her blending into human settings. She wields twin Colt M1911 pistols with proficiency against Shikabane.

Posing as a transfer student with Takamasa acting as the school nurse, they infiltrated Ouri Kagami's high school. This guise allowed Itsuki fleeting participation in ordinary school life, partially fulfilling her yearning. During this mission, she confronted the rogue monk Akasha Shishidou while rescuing Ouri, exposing her true role within the Kōgon Sect.

A poignant chapter involves her former friend Tomoharu Kishibe. He perished in a separate car accident caused by reckless teens, later returning as a Shikabane. Four years on, near the second season's conclusion, Itsuki and Takamasa terminated him, resolving this trauma from her human existence.

Over time, mutual romantic feelings developed between Itsuki and Takamasa, yet remained unacted upon due to her undead state and monastic codes governing their duties. Their relationship highlighted the tension between personal bonds and Kōgon Sect principles, with Takamasa using their dynamic to instruct Ouri on prioritizing monastic obligations over emotional ties.

Itsuki's name combines "different" (異) and "moon" (月) for "Itsuki," while "Yamagami" means "mountain god" (山神). Personal details include her O blood type, March 18th birthday, dislike of spicy foods (excluding curry), fondness for vanilla ice cream and pasta, and proficiency in artistic gymnastics.