Mari Nishino is Takeru Takemoto's cousin. After Takeru's mother abandoned him, Mari's parents, Itcho and Sumie Nishino, welcomed him into their home. The Nishino family operates both a delivery service and a boarding service. Mari lives with Takeru, her parents, and later, Hikari and Jennifer Portman.

During the early series, Mari frequently nags Takeru about his behavior and lifestyle. Beneath this outward criticism lies jealousy over Takeru's deepening connection with Hikari, the mysterious girl he discovered. Mari initially denies her own feelings for Takeru but gradually acknowledges them as the story unfolds. Her distrust of Hikari escalates alongside her unrequited affection for Takeru, heightening the emotional tension within their shared household.

Approaching the climax, Mari openly confesses her love to Takeru in an effort to deter him from pursuing Hikari. Takeru responds with a grateful smile yet chooses to follow Hikari, leaving Mari's feelings unresolved and emphasizing the one-sided nature of her romantic attachment.

Mari attends high school and possesses dark green hair and olive eyes, often styling her hair in pigtails. Her interactions predominantly center on the central love triangle involving Takeru, Hikari, and herself, defining her role as a childhood friend navigating unreciprocated emotions.