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. According to Aigis, he considers the shrine a sacred place.

Koromaru also exhibits numerous human characteristics, despite being a dog. He exhibits intelligence far above that of most dogs. He is fiercely loyal, and particularly close with Shinjiro Aragaki, who often feeds him special food and takes extra care of him. He is a fan of a show called "The True Battles of Real Men," which exhibits acts of courage from real people. He begs the protagonist to take him to the theater for a special showing of the final episodes of the show. Koromaru whines and runs around after the movie until the protagonist agrees to purchase the boxed set after its release.

In Persona Q: Shadow of the Labyrinth, even though he gets along with everyone, he dislikes Kanji Tatsumi for some unexplained reason and refuses to let Kanji pet him. Of all (surviving) members of SEES, he seems to be the closest to Ken, judging by his decision to move in with him as shown in Persona 4 Arena Ultimax.

No preference