Haru Matogi, a dedicated high school student, diligently pursues his dream of becoming a veterinarian who understands animal language. This focus leads him to prioritize studies over non-academic pursuits, often overlooking time with his childhood friend, Miku Mizuki. His profound care for animals manifests in his responsibility for the school rabbits kept on the rooftop, showing special affection for one named Mimika.

Haru becomes pivotal when Mikami Inaba, the school's toughest fighter, is distracted by his arrival during a rooftop confrontation. This distraction causes an accident where both Mikami and Mimika plummet from the building. As they fall, Mikami grabs Mimika, triggering a mysterious pink light that fuses their bodies and personalities. Haru takes responsibility for the merged being, which primarily exhibits Mimika's cheerful and affectionate personality, though Mikami surfaces in response to threats.

Oblivious to romantic feelings, Haru fails to recognize Mimika's open, physically affectionate gratitude for his past care or Miku's longstanding unrequited feelings and resulting jealousy toward Mimika. Initially helpless in danger, Haru nonetheless displays fierce loyalty and protectiveness, consistently attempting to defend Mimika and his friends against adversaries like the dog-merged Chou or the cyborg Dekao, even when outmatched. His typically reserved, studious nature contrasts sharply with the intense emotions surrounding him. The fusion experience, however, fosters subtle personal growth, deepening his understanding of Mikami's traumatic past and her appreciation for his genuine treatment of her as a person.