Soma Mizuno is a firefighter characterized by initial womanizing tendencies and a casual approach to relationships. As Ryo Fujihashi's childhood friend, their bond drives his core development. Despite his flirtatious reputation, he demonstrates inherent seriousness and responsibility in his professional role.

When rescuing Ryo from her burning apartment, he acts on his commitment to saving lives. He subsequently offers her shelter at his home after she loses her residence. Their proximity during this period gradually transforms their friendship into romance. Soma's early advances toward Ryo align with his flirtatious persona, yet deeper feelings surface, revealing a longstanding romantic interest distinct from his superficial interactions with others.

His evolution centers on shifting from casual flirtation toward authentic emotional commitment. His persistent affection reignites mutual romantic feelings with Ryo, exposing underlying sincerity beneath his playboy facade. Throughout this personal growth, his unwavering dedication to firefighting remains consistent. His personality merges apparent frivolity with genuine depth, particularly in protective instincts and eventual devotion to Ryo.