Koromaru's master was killed in an incident involving Shadows. He became a stray, and was watched over by Fuuka and Yukari. Instead of attacking with his fangs or claws, Koromaru wields daggers in his mouth. His Personas can be used to cast fire spells, as well as darkness-based spells that occasionally instantly kill the enemy. He has a speed advantage over the other characters.

Koromaru remains faithful to his master even after his death, retracing the path of their old walks and revisiting the place where he was slain. One day when Akihiko is hunting Shadows, he comes across the canine, who is gravely injured. The Shadows surrounding him are dead. Once Koromaru is healed, Ikutsuki reveals that he too has the potential to summon a Persona. Mitsuru gives him a special collar-like Evoker and he is inducted into SEES.

Aigis claims to be able to translate Koromaru's barking and body language into human speech.