Setsu Yuuki, sometimes localized as Dylan Yuki, is a central human character. He is a 13-14-year-old honor student known for his quiet, bookish demeanor, often seen reading small books. His personality blends intellectual reserve with underlying loyalty; he readily assists friends despite occasional embarrassment and possesses a notable aversion to green peppers.

His journey is defined by romantic tension. Both Kaede Minami (Katie) and Azumi Hidaka develop early crushes on him, while his own feelings remain initially ambiguous. He dates Kaede during the anime's first season, with the narrative presenting whether he initially preferred Kaede or Azumi as unresolved. His relationship with his fairy partner, Rirumu (Rima), is pivotal; Rirumu actively supports Kaede's romantic pursuits and occasionally clashes with Setsu, including an argument preceding his key emotional realization.

The fourth season introduces Haruka Morishita, his childhood friend. Haruka confesses her strong love, leading Setsu to gradually grow closer to her. However, Haruka later recognizes that Setsu's true feelings are for Kaede and selflessly releases him, urging him to act before losing the person he truly desires. Following the argument with Rirumu, Setsu retreats into deep contemplation at home. This introspection culminates in his decision to call Kaede, requesting a meeting at a park and asking her to bring a watch.

During this park encounter, Setsu openly shares his feelings and confesses his love to Kaede, who accepts. This confession and mutual acceptance resolve his primary romantic arc within the anime. He functions as a grounding presence within the group, often serving as the "Only Sane Man" amidst the chaotic dynamics involving fairies and human relationships. His background establishes him as Kaede's classmate and best friend, contextualizing their evolving connection.