Despite his young age, Ken is very intelligent and mature. It is shown that this is a coping mechanism due to the death of his mother at a young age, which more-or-less forced him to give up his childhood. His maturity is noted by Yukari, who can't believe he's in elementary school and claims that Junpei, who is a full six years older than him, could learn a thing or two from him. Fuuka also notes his maturity, and finds him cute as a result of it. Besides being mature, Ken also knows several words that boys his age don't usually know, such as "bastard," which he usually uses when referring to Strega. He is very polite to others, and never misses a chance to show respect where respect is due. He is also honest, diligent, and deeply caring towards his peers, particularly the members of SEES. Ken is shown to like and care a great deal for animals, such as Koromaru. In his video recording, there is a hamster in his room. In Persona 3 The Movie, he cares for a goldfish and in Persona 4 Arena Ultimax, it is shown that he and Koromaru are now living together.

Although Ken is very mature, he still has several qualities that fit with his age and is sometimes too precocious for his own good. He finds a role-model in Akihiko, whom he admires for his boxing and fighting talents, often referring to him as "Sanada-san" as a form of respect to his senpai. He also likes to watch a children's show, Featherman R, but he prefers to keep it a secret. He seems to collect a lot of the Featherman R figurines, as well. He is also shown to be quite naive at times, such as when Junpei tricks him into believing the girls aren't wearing anything under their kimonos, which they wore on New Year's Day. Overall, Ken gives off the impression that he's just trying too hard to be something he's not i.e. an adult. Ken notes that one of the reasons he uses spears in battle is so that he'll be on even ground with an adult.

He has a strong dislike towards milk, but he forces himself to drink it because he believes it can make him become taller. Unfortunately, it seems he may be lactose-intolerant since it often makes him sick. In the female protagonist's route, he acts shy towards her, due to the crush he develops on her if the right choices are made during his Social Link.

During the October Full Moon Event, his reason for joining SEES is revealed: he confesses to Shinjiro Aragaki that he is lonely and vengeful, desperate to avenge the killing of his mother, the killer turns out to be Shinjiro. In The Answer, it is shown that he attempted to tell the truth about his mother's death to a police officer, but the officer dismissed it as some hallucination Ken had as a result of trauma. This convinced Ken that he couldn't rely on adults, and so resolved to track down his mother's killer on his own. Ultimately, he became completely consumed by his desire for revenge, to the point that it has become his one and only goal in life, and he even implies that he plans on killing himself once it's done. In Midsummer Knight's Dream, however, he initially thought it was just some random Shadow that killed his mother and only discovers that it was Shinjiro later on, ultimately attempting to kill him out of a feeling of betrayal. Ken is portrayed as being more polite and having more remorse over Shinjiro's death in the Japanese version compared to his portrayal in the English version. In fact, in the Japanese version, it is made very clear that Ken feels remorse by outright saying that he no longer wants to kill Shinjiro anyway. The English version, however, takes a more subtle approach to the scene where he laments over the fact that he can no longer be the one to take revenge on Shinjiro while crying afterwards, suggesting that he is somewhat dishonest in the English version.

No I do not have a preference, make sure he sounds like a kid, girls and boys are welcome to try!