Tsutomu Yamaguchi grows up as the childhood friend and neighbor of Mikako Kouda, their families sharing the same Tokyo apartment complex since infancy. He enrolls at Yazawa Art Academy to study visual arts, initially experimenting with abstract art made from found objects while remaining uncertain about his future career path. Tsutomu possesses a tall frame, brown hair, dark brown eyes, and a handsome appearance, earning him the nickname "Monkey" from Mikako. His striking resemblance to Ken Nakagawa, the popular singer from Manbou, frequently leads to mistaken identity. This likeness attracts unwanted attention at school, including love letters, autograph requests, and being chased by fans, causing social complications.

Generally laidback, accommodating, and kind, Tsutomu's nature sometimes makes him vulnerable to manipulation, such as briefly dating Mariko Nakasu due to peer pressure. Despite his easygoing demeanor, he exhibits protectiveness and jealousy regarding Mikako, especially when other boys show interest in her. Their relationship gradually evolves from platonic friendship to romance, characterized by frequent arguments followed by reconciliations through open communication. Tsutomu co-founds the street market club Akindo with Mikako and their friends, supporting her fashion brand Happy Berry while slowly gaining confidence in his own artistic direction.

His interest shifts towards photography, significantly influenced by Mikako's father, Hirohiko Sakurada. This passion solidifies into his chosen career. After marrying Mikako and having a daughter, Tsutomu works as a professional photographer. His established profession is later confirmed through a cameo appearance in the related series *Paradise Kiss*. His character arc concludes with him successfully balancing his creative career with family life, marking his growth from initial artistic indecision to professional fulfillment.