Saiko Yonebayashi, a 143 cm-tall Rank 2 Ghoul Investigator, possesses a stockier frame than typical CCG agents, with pronounced chubby legs. Her light pale blue hair initially falls in unevenly fringed pigtails, later shortened post-Tsukiyama Operation. Heterochromatic eyes—bright baby blue right and vivid red left—signify her Quinx status. The younger of two siblings, her childhood was marked by familial upheaval after her mother relocated the family to live with a lover, prompting financial strain. This led Saiko and her brother to attend Academy Junior School for its affordability, where she prioritized video games over studies, frequently clashing with instructors. Initially disinterested in investigative work, she became the top Quinx Aptitude Test candidate, pressured by her mother to undergo the procedure for its monetary benefits. As a Quinx, her enhanced physicality and rinkaku kagune first manifest as twin reptilian-tail tentacles, later evolving into a towering insectoid structure with unmatched destructive force. Though initially draining her stamina to vulnerability, post-Tsukiyama mastery allows efficient manipulation into multiple tentacles or the colossal form, enabling temporary suppression of Koutarou Amon’s kakuja. Introduced as a lethargic junk-food enthusiast who skips missions due to oversleeping, Saiko gradually embraces her deputy squad leader role, mentoring new recruits and bolstering Urie’s leadership. Her relationships reflect nuanced growth: a parental bond with Haise Sasaki (“Maman”), persisting despite post-departure distance; playful teasing mixed with genuine concern for Shirazu; a friction-turned-partnership dynamic with Urie; and protective relief over Mutsuki’s safety. Interactions with Mado, Ui, and Kuroiwa further underscore her integration into CCG operations, illustrating her transition from apathy to committed responsibility.