Yasumi Natsu, younger brother to Sunao Moriyama's classmate Mikan Natsu, possesses a notably taller and more mature physique than his sister and peers, often appearing older despite his age. His name forms a pun on the Japanese term "natsuyasumi," meaning summer vacation.

He exhibits a protective and responsible demeanor toward Mikan, frequently disrupting her attempts to spend time alone with Sunao due to her intense crush. His interventions stem from both annoyance at her infatuated behavior and genuine protectiveness, often physically pulling her away from Sunao.

Beneath his stern exterior, Yasumi displays compassion, particularly toward the small creature Potemayo, who returns his affection—sometimes to his exasperation.

Background hints suggest complex feelings toward Mikan beyond typical sibling concern. After accidentally touching her breast, he retreated to the bathroom for an extended period, showing emotional disturbance. He also experienced a nosebleed—an anime trope for intense reactions—after imagining her naked, indicating internal conflict.

As a secondary character, Yasumi primarily complicates Mikan's romantic pursuits, provides comic relief through reactions to her antics, and reveals moments of vulnerability or compassion.