Note: Tohru probably should be romanized as "Toru", but all the information I found uses the 'h' romanization.
Birthday: 1st of May Year 3, class C
As the male lead in the OAV, Tohru doesn't have any peculiar traits, but due to time constraints has little room for character development. Some time ago, Tohru rescued Mizuho's little brother Tatsuya from a river. Since then he's been acquainted with the family, and is much like a big brother to Tatsuya.
In the past Mizuho was depressed about her tennis performance, and Tohru gave her a key chain and encouraged her. While she's always remembered him for his kind words, their relationship has been platonic, even if her heart suggests otherwise.
While not above checking a girl out, Tohru is generally respectable with decent morals. He's reasonably polite, but hostile towards Haruhiko.
Year 3, class C
As TV series more closely follows the game script, the male protagonist Tsuyoshi likewise shows traits consistent with what was implied in the game. Notably, Tsuyoshi is a notorious skirt chaser, earning him a reputation with the girls around school. Even so, he's not unpopular with the ladies either. His behavior in the presence of a pretty girl suggests it's at least partially true, but he's never lewd or unscrupulous (like Haruhiko who is in many ways his opposite).
He's laid back and easy going. While there's an ongoing rivalry with Haruhiko, Tsuyoshi doesn't seem to harbor any ill will towards him.
Year 3, class C (classmate)
Minoru is very much the "best friend" guy found in date sims. Good natured, fun loving, talks a lot, and isn't above ogling girls with the protagonist. Academically his performance is poor, likely due to spending his energy on romantic pursuits.
He's particularly interested in Minatsu, the waitress working at the Dogeza Cafe; thus spending much of his time there. As Minatsu is a romantic option, that can turn into a rivalry, however if you don't raise her opinion high enough by a certain point, you lose her to Minoru.
He plays a smaller role in the OVA, but regularly appears in the TV series.
Year 3, class C (classmate)
Haruhiko is the general antagonist of Kakyuusei. He comes from a rich and powerful family, and acts the part. It is however an act, and deep down he's a sinister person; especially in regards to girls. Haruhiko fancies himself a smooth player, but most girls find his lines cheesy and only humor him. You can tell a lot about a girl's personality, by how they regard Haruhiko, although only Reiko seems to know him well. While they're acquainted and run in the same circles, he never makes a move on her. Probably because he's not able to leverage his wealth and influence in the predatory manner he'd prefer.
In the OVA Haruhiko sets his sights on Mizuho. As both are in the tennis club, they're near each other often. Mizuho regards him favorably, but in a very platonic way. Tohru isn't happy about this, knowing Haruhiko's true nature.
In the TV series Haruhiko pursues a few girls, but has no success with them. He believes Tsuyoshi is the culprit behind his failures, although the reality is that it's more in his head. Tsuyoshi doesn't like Haruhiko, but doesn't particularly hate him either.
In the game, he's a nerdy and somewhat sickly underclassmen that looks up to you. He plays a key role to winning Shizuka in the game. In the TV series he conveniently offers pertinent information at just the right time.
He's Tina's brother. Although similar in appearance, he doesn't float around like his sister. He's very serious, and dislikes you in the game. Nana grows to have a crush in him, only to have her heart broken when she confesses.
They guy who runs the Dogeza Cafe, although his actual name is never stated. He has a crush on Miyuki, and is a part of a double date with Tsuyoshi and Ai in the TV anime. He's an older guy, but said to be strangely pure hearted. Although Miyuki doesn't think of him in the romantic sense, she comes to appreciate his thoughtfulness and kindness. Whenever she visits the cafe, he treats her to a large desert.
Miko lives at a Shinto shrine with her father. He's pretty much the terror every high school boy worried about when dating a girl. He's not only protective of his daughter, but also traditional to the point where it's like he's from another era entirely. He becomes especially agitated if boys refer to his daughter as "Miko-chan" (implying familiarity) instead of the properly distant Kamiyama-san.
birthday: 22nd February occupation: seafood shop employee measurements: 83-61-89 (cm) blood type: A height: 154 (cm)
Actually, there's one girl I didn't mention... Maki has short purple hair, and works at a fish monger shop. She inherited the business after graduating college, and is known for her loud voice at the storefront. She doesn't make any appearance in the OAV, with a small role in one episode of the TV series.