Masago Matsuo is the main male protagonist's best friend. He has dark blue hair (also described as black) reaching his neck and gray eyes, fitting a teenage design, and typically wears a school uniform with a tie.

He possesses extensive knowledge of the tsundere behaviors displayed by both his best friend and the protagonist's younger sister, Chiho Kagari, acting as an observer who understands their contrasting romantic approaches. His primary hobby involves dedicated play of gal games (dating simulations), often using the television at his best friend's house.

This gaming habit frequently causes friction with Chiho, who becomes visibly frustrated when he occupies her living space for long sessions. Their dynamic includes her physically reacting, such as swatting at him. Despite Chiho's developing romantic feelings for him, which her own tsundere nature makes difficult to hide, he remains largely oblivious, his focus predominantly fixed on gaming rather than noticing her emotional shifts.

His role consistently centers on facilitating comedic situations through his gaming obsession and serving as a supporting figure within the main friend group. Interactions occur primarily within school settings and the protagonist's household.