Koromaru is a dog. With human-level intelligence that can summon a persona. It’s barely explained. In fact, for that matter, it contradicts other information given about personae, in that the whole reason Aigis is shaped like a human and not a tank or something is that it’s necessary for her to see herself as human in order to be able to manifest a persona, but here’s this dog with a persona so whatever.
Anyways, Koromaru is basically Hachiko. He used to live with a monk that would take him on nightly walks. One day, the monk got killed by shadows, and Koromaru continued their nightly walks alone, and spent a lot of time hanging around the shrine the monk used to take care of. Then one day, the team detects some shadows roaming outside Tartarus attacking the shrine. Akihiko heads there, only to find the shadows already dead and Koromaru injured. Putting 2 and 2 together because he’s not the dumb meathead the later games think he is, Akihiko realizes this dog must have a persona. So he takes him back to the team, they get him medical care, and then Koromaru joins up with S.E.E.S.
Every Persona from this point forward has an animal-like character. Persona 4 and 5 makes sure its a character that can talk, however. Koromaru can’t, which poses some problems. To be fair, they do a decent job of communicating Koromaru’s personality through his actions, such as it is, and Aigis can understand his thoughts and sometimes interprets them for others. From that, you learn that Koromaru is very loyal and protective of those he considers family, and admires bravery. He also likes certain TV shows demonstrating brave people, and has a near human-level comprehension of the world. But they only take it so far. Koromaru gets left out of a lot, not having much input in dialogue heavy scenes, not usually being with the party when they’re away from the dorm, and not really having much in the way of impact on the plot. Which is a shame. We only get a shallow view of Koromaru, and there was a lot more room for developing him. As a result, he’s the most forgettable member of the cast, to the point he either doesn’t get included or gets bound together with another character for most of the spinoffs.
He does have one big moment, however. In the midst of the party’s darkest moment, when Shuji Ikutsuki betrayed them and was in the midst of crucifying them, he didn’t bother crucifying the dog. It was Koromaru who tore the device he was using to control Aigis away from him, enabling her to break free from her programming and save the party. So, if it weren’t for Koromaru, the party wouldn’t have survived to save the day and make millions of dollars in game sales. That counts for something, at least.
In combat, Koromaru’s speedy. The most speedy. He’s the fastest, most accurate, and most evasive character in your party, and tops most shadows in all those areas, too. On the flip side, he’s really not durable. Shadows will have a harder time hitting them than anyone else, but when they do, he’ll go down fast. Other than that, not especially much to write home about. He uses knives to fight, and is accurate but not so damaging with them. He does pretty decently with magic, and has an arsenal of fire and instant-kill darkness spells behind him. Given the only other character with dedicated fire spells is Junpei, who is realllllly not great at magic, he’s the one to go with if your MC’s personas are focused on other things. He’s also a very direct character. Doesn’t have a lot of tricks to him, pretty much just basic attacks and direct damage or instant-kill spells. His persona is Cerberus, which both fits his doggy nature, and I’m also pretty sure is a reference to Pascal, your dog that you turned into Cerberus in SMT 1. He does not get an ultimate persona. That requires personality development, and when you barely show up in the plot, well…
Nobody likes Ken.
What? No. No, we’re not doing that. No Ken. I’m not going there. You can’t make me.
Ok, so Ken is this little prat that nobody actually wants around because he ruins everything he touches. And that’s about all there is to say about Ken. Let’s move on.
Seriously. There are things mankind is not meant to know. The existence of Ken Amada is one of those things. Trust me.
You don’t trust me.