I think the word that describes Kotarou best is “human”. Despite being different from his peers, he tried to fit in. He was afraid of getting isolated, afraid of exposing his true nature, so he put on an act and pretended to be an easy-going, light-hearted high schooler who loved to joke around. Quite successfully at that, since he really could ‘get along’ with just anybody.
His desire of a normal, fulfilled youth, as he described it in the monologue at the very start of the game, was very normal and truly human-like.
On the inside, Kotarou was a half-assed coward who was afraid of being hurt - another human-like feature of his.
However, what made him most human-like, in my opinion, was how he changed throughout the routes, depending on the girl he chose to follow and the position he ended up in. Unlike pretty much every protagonist who would behave as “the same person in different situations” in different routes, he became a truly “different person” as his character developed depending on his environment, which is, again, very human-like. When their environment changes drastically, humans change along with it.
Those different ‘paths’ that his personality has followed in different routes are quite extreme if compared with each-other. In Kotori’s and Shizuru’s routes, Kotarou was “weak” (which is not a purely negative trait), in Chihaya’s route, he was “righteous and idealistic”, in Lucia’s route he was “dedicated and somewhat dense”, in Akane’s route and Terra, he was “merciless and realistic” while in Moon, he had… a part of everything, I guess?
The Kotarou I liked most was the common route Kotarou.
Afraid of being shunned by his peers. Being half-assed about everything and unable to find something he could be passionate about and really give his all for. Trying to fit in yet always remembering that he’s different from the others, thus (unintentionally) creating a gap between himself and most people. Wishing for a good, fulfilled life. And lastly, acting really hilariously on a daily basis, always creating funny situations that make the common route very enjoyable to read. I can really relate to him because of these traits of his and I sometimes wish I was as talented in terms of comedy as he is ^^
As a Key protagonist, he’s definitely among the best. He is the kind of character that wouldn’t feel out of place no matter what situation you put him into. He would do very well in both an ordinary, carefree school life situation, having fun with his friends as well as on a bloody battlefield where he would have to fight in order to protect what’s precioius to him.