Seiji Yagiri, a Raira Academy student and Namie Yagiri’s younger brother, harbors an all-consuming obsession with Celty Sturluson’s disembodied head, first encountered in his uncle’s study during childhood. This fixation stems from a pivotal moment when Namie revealed the preserved head, igniting romantic longing that drives him to steal it from Yagiri Pharmaceuticals and relentlessly pursue reunification.

During his teens, he defends Mika Harima and Anri Sonohara from harassment, unintentionally sparking Mika’s infatuation. Her obsessive stalking leads her to discover the hidden head, triggering his panicked attack to silence her. Believing her dead, he enlists Namie to conceal the incident, unaware Mika survives and undergoes surgery to mimic the head’s appearance. Though he initially overlooks her transformation, he gradually forges an unconventional bond with her, even as his fixation on the original persists.

Moral contradictions define him: he intervenes chivalrously against harassment yet grapples with guilt after nearly stabbing Mikado Ryuugamine. Resourceful in conflict, he wields improvised weapons like pens against adversaries such as Shizuo Heiwajima, showcasing combat agility.

Family dynamics further complicate his life, marked by Namie’s unreciprocated possessive and incestuous attachment. He exploits her emotions, feigning intimacy with a finger between their lips to deter her from targeting Mika. Their isolated upbringing under neglectful parents and an absent uncle fuels Namie’s warped devotion.

Later, he and Mika join the Dollars gang, maintaining dual lives as students. After Namie flees with the head, they plan American studies, tracking it to Chicago. His journey evolves from singular obsession to tempered determination, balancing lingering desire for the head with loyalty to Mika. Traumatic past and tangled relationships forge his paradoxical nature—a blend of destructive fixation and protective resolve, enduring even as Celty pauses her search.