Setsu Yuuki is a tall, handsome middle school student with brown hair and blue eyes, typically maintaining a reserved expression. His physique evolves from slim to more muscular throughout the series. Recognized as an honor student, he excels academically and is frequently seen reading small books, occasionally standard-sized ones. He possesses notable athletic ability, particularly in basketball, and aspires to become a novelist. He harbors a strong aversion to green peppers, verging on a phobia.

Initially quiet and introverted, Setsu appears aloof, especially towards classmates Kaede Minami and Azumi Hidaka, both of whom develop crushes on him; he finds their rivalry for his attention annoying early on. He gradually reveals a loyal and caring nature, becoming more willing to assist friends despite occasional embarrassment. His fairy partner, Rirumu, actively supports Kaede's feelings and intervenes in their relationship.

Setsu's romantic journey is significant. He briefly dates Kaede in the first season while ambiguously appreciating Azumi. Later, his childhood friend Haruka Morishita reappears, confessing her love and temporarily growing closer to him. Haruka eventually steps aside, recognizing Kaede's importance, prompting Setsu to confront his feelings. This culminates in a decisive park confession to Kaede in the fourth season; after a heartfelt conversation, she accepts, and they become a couple, resolving the love triangle as Azumi concedes defeat following a challenge.

His relationship with Rirumu involves minor conflicts, including an argument preceding his emotional confession to Kaede. He interacts with other fairies like Mirumo during group adventures, though Rirumu remains his primary partner. Setsu's growth is marked by increased emotional openness and decisiveness, particularly in pursuing his genuine affection for Kaede.