Koromaru is a white shiba-inu with red eyes. When he joined S.E.E.S, he was given a special Evoker for dogs, metallic collar around his neck. He has a red armband on his left arm, a miniature, white T-Shirt, and very small angel-esque wings on his back.
According to Shigenori Soejima, the idea of including a dog into the party was from the very beginning so the hard part was settling on a breed. Small dogs like Chihuahuas or Pomeranians couldn't be used because of their size, so the staff held a popularity poll to decide on its type. Dobermans would look too scary, so the final decision was between a Husky and a Shiba, although at first Soejima chose a Dalmatian. The poll decided on a Shiba, but a normal dog would look too plain, so the next move was choosing some clothes. Being a shinto priest's dog, he thought of giving him the same garments. The wings attached to his costume were Yukari and Fuuka's idea.
Koromaru's personality is based on the famous dog Hachikō. He is very kind and has shown extreme loyalty, choosing to stay at his master's shrine long after his death and going for a walk everyday 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.
- Koromaru appears to be based off Hachikou, a famous dog in Japan. Hachikou saw his owner, a Tokyo University professor, leave for work every day, and then he would wait for the professor at the train station in the evening. His owner died from a cranial hemorrhage while giving a lecture, but Hachikou continued to appear at the station, waiting for his owner to return. Hachikou appeared every night when the trains would arrive, hoping to see the professor. The dog's loyalty to his master caused him to return to the Shibuya station every day for almost ten years, something which inspired and impressed others. When Hachikou died, a statue was erected at the station in his memory, and Hachikou's remains were stuffed and placed in a museum.
- Aigis is the only one who can "understand" what Koromaru has to "say". Oddly, once Aigis awakens to the powers of the Wild Card in The Answer, she understands less and less whatever Koromaru tries to tell her. It seems that, by becoming more "human", Aigis also loses some of her non-human traits.
- Persona 4's quest #20 "We Wish Our Dog Would Return" is about helping the dog named Koro. However, it is a different dog.
- When Aigis beats Yosuke in Persona 4 Arena's versus mode, Aigis remarks that because he resembles Koromaru because of his speed and adds that if he barked, he'd be exactly like him.