Koromaru's personality is based on the famous dog Hachikō. He is kind and has shown extreme loyalty, choosing to stay at his master's shrine long after his death and going for a walk every day like he used to with his master. He is also very protective of the shrine, as shown when he risked his life to defend it, but got seriously injured in the process.

Koromaru also exhibits numerous human characteristics, despite being a dog. He exhibits intelligence far above that of most dogs.