Yui Minamito is Junpei Manaka's childhood friend and former neighbor who moved away before later reconnecting. Approximately one year younger than Junpei, she interacts with him in a sibling-like manner. She often acts as an overprotective figure, particularly when others like Satsuki Kitaoji display romantic or clingy behavior toward him, a dynamic stemming from their shared past where Junpei was perceived as a crybaby. Recognized as one of the four main heroines, she does not develop romantic feelings for Junpei nor seeks a romantic relationship; her interactions remain strictly platonic and familial.

She attends Oumi Academy, admiring Tsukasa Nishino as a role model and developing an infatuation with another student named Okusa. Yui possesses an unintentional habit of sleeping naked, often leading to situations where she is found unclothed in Junpei's bed during sleep. Her personal preferences include a fondness for chocolates and cake.

Her strict parents, particularly a father with whom she has conflicts, form her family background. After arguing with her father over living arrangements, she moves into Junpei's family home temporarily. Later, she decides to live independently in her own apartment, facing challenges with daily tasks like cooking. Despite this move, she continues visiting Junpei's home and occasionally sleeps in his bed when given the opportunity.

Yui is associated with the cardinal direction "South," indicated by the first kanji in her surname, Minamito. Her role focuses consistently on her supportive, sisterly bond with Junpei across media, without significant evolution or involvement in central romantic plotlines.