Masakatsu Yuki attends Sakaimachi Academy as a second-year student, junior to Takuya Arima and Mio Shimazu. His given name Masakatsu blends "proper" (正) with "victory" (勝), while his surname Yuki fuses "tie/bind" (結) and "castle" (城). He habitually wears identical parkas in rotation, claiming multiple copies for daily changes. This parka transitioned from plain in the original game to lettered in the remake, and his signature accessory shifted from a hat to a hairpin.

Masakatsu idolizes Takuya Arima as "Boss" after being saved from bullies, seeking Takuya’s guidance as a role model. Though outwardly cheerful and sociable, he battles timidity and is easily swayed, especially by girls. While often a source of comic relief, he nurses unrequited feelings for Mio Shimazu. His attempts to impress or confess to her consistently fail due to Mio’s romantic interest in Takuya, fueling his frustration.

A pivotal moment occurs when Masakatsu overhears Mio and Takuya discussing her mayor father’s corruption at the beach. Hoping to win Mio’s affection by exposing the truth, he posts incriminating flyers on the school bulletin board. This backfires catastrophically: Mio feels profoundly betrayed by the public revelation, prompting her to leave for studies in America. Masakatsu drowns in regret and guilt over causing her pain and departure, leaving enduring emotional scars.