Masakatsu Yūki attends Sakaimachi Academy as a second-year student, frequently interacting with upperclassmen Takuya Arima and Mio Shimazu. His surname blends "tie/bind" (結) with "castle" (城), while his given name merges "right/proper" (正) with "victory" (勝), thematically underscoring correctness and triumph.

He wears a signature parka daily, insisting he owns multiple identical copies. His design evolved slightly between original and remake iterations, exchanging his original hat for a hairpin and adding lettering to his previously plain parka. Kind-hearted yet timid, Masakatsu often provides comic relief through sociable but impressionable behavior, especially around female classmates.

His past includes a pivotal incident where Takuya Arima saved him from bullies, forging Masakatsu’s unwavering admiration. He subsequently addresses Takuya as "Boss," seeking guidance and emulating his actions. Simultaneously, Masakatsu nurtures unreciprocated feelings for Mio Shimazu, repeatedly attempting confessions despite her clear preference for Takuya, leading to consistent rejection and frustration.

A turning point occurs when Masakatsu overhears Mio and Takuya discussing corruption involving Mayor Shimazu at the beach. Hoping to shift Mio’s perception of him, he exposes the scandal by posting flyers on the school bulletin board. This backfires catastrophically, wounding Mio deeply with betrayal instead of drawing her closer.

The fallout drives Mio to depart for studies in America. Masakatsu endures profound regret and guilt over his actions, which permanently fracture his bond with Mio and leave enduring remorse. His arc demonstrates how rash attempts to manipulate relationships yield irreversible personal and interpersonal damage.