Koromaru is certainly not your average party member. In fact, he’s not even human. This addition to the group in Persona 3 is a hyper-intelligent canine, who even has the ability to summon the Persona Cerberus.
The pup has a rather tragic past though, as his master, who was a monk from Naganaki shrine, died in an accident six months before. Koromaru stoically protects the shrine until a Shadow wanders in one night and is killed by the dog. From then on, he joins SEES, and gets an evoker allowing him to summon his Persona at any time.
Koromaru is fiercely loyal to the group and protagonist, oftentimes comforting characters and protecting them. Cerberus gives him a big buff with fire abilities, and his speed can be a huge boon in battle.