Large Ham: Whenever she passionately starts talking about honor and blood brothers and such.
Love Can Make You Gonk: Kumiko does this whenever she is fawning over Shinohara. Her eyes go from their usual fixed glare to big, sparkly, bishojo-standard and can flick back and forth between panels when the situation calls for it.
Jerk with a Heart of Gold: Shin is very aloof and cold, but he goes to great lengths to help and protect his friends. He even saves Kumiko a few times, though that's arguably after the probable time he started developing a crush on her.
Red-Headed Hero: Shin's dyed red hair, as well as his enrollment in Shirokin, is a symbol of his disdain for his father's oppressiveness. His hair's color and shaggy quality earn him the nickname "Young Master Red Lion" from Kyo.
The Stoic: Shin very rarely changes his expression of casual disinterest for anything or anyone.
So when he shows anger when he hears Kimiko has been tricked to get ambushed by her students, you can see why OOC Is Serious Business.
Surrounded by Idiots: He was able to convince his friends that his bruises and cuts were from a cupboard falling on him after a huge earthquake... until Minami points out that there hadn't been an earthquake recently.
Teruo "Kuma" Kumai
Shin's best friend and classmate since middle school.
The Big Guy: His nickname, "Kuma," means "bear" in Japanese.
Book Dumb: He somehow managed to get a zero on his math test.
Even Bad Men Love Their Mamas: More rough around the edges than bad, but Kuma really loves his mother. There was even an arc in the manga devoted to Kuma considering dropping out of high school to take care of his mom, with his whole class pitching in to help so he wouldn't have to.
Papa Wolf: When Kumiko comes home extremely ill after being kidnapped by Kudou, Ryuichirou orders the destruction of the entire Nekomata kumi. The kumi had enough sense to scatter before the Kurodas came after them.
Non-Action Guy: Shinohara is the only man in this series who can't hold his own in a fight. He got in trouble with some gangsters he was still in college and had to be rescued by Ryuuichirou, Kumiko's grandpa.
Obfuscating Stupidity: You may think Shinohara completely doesn't notice Kumiko's crush on him. You would be wrong. He has his reasons, though.
Big Brother Mentor: He acts as this toward most of the Kuroda group, but especially Kumiko. He was the one who taught her how to fight. He also tries to be this to Shin, though Shin is strongly against it.
Dark and Troubled Past: During her previous teaching job at a middle school, one student attempted to commit suicide due to bullying, and Fujiyama, as his teacher, was blamed for not noticing it. She went on an aimless journey (i.e. she took a vacation), and since then has been determined not to let the same thing happen to one of her students again.
Ephebophile: She jokes about it, but she might not be serious. She does like her boys young and thin, though.
Boxed Crook: A variation; After he fails to kill Kumiko a third time, he's captured and brought back to the Ooedo compound, where he's kept as a servant. This is to reform him, and he actually does get a lot better because he realizes the Kurodas are keeping him in to protect him from all the people he's angered.
Non-Giving-Up School Guy: He's desperate to make Shin take the entrance exams for prestigious universities in order to raise Shirokin's reputation. He's apoplectic when he hears Shin doesn't want to go to college.
Vice-principal: FINE! I'LL GET A HOLD OF HIM AND PERSUADE HIM MYSELF! *leaves office* Shawada-kuuuun! <3
Fujiyama: Oh my... Poor Sawada Shin.
Kumiko: It's all right. He won't be caught.
Kumiko's dog. Half Tosa dog, half... horse?
Ascended Extra: He shows up for some silliness in bonus chapters and occasionally provides some help in the manga, but in the anime, he shows up every single episode and even has some important acting and speaking roles (though no one can understand him except the audience).
Funny Animal: In the anime, it's almost as if he's a human in a dog's body.
Half-Dressed Cartoon Animal: In the special chapters of the manga, he wears a sweatsuit jacket similar to Kumiko's, though when he shows up in the main story, he's naked except for the ribbon on his tail. In the anime, he wears the jacket all the time.
Interspecies Romance: It's unknown who Fuji's father is, but judging by his tail, many have presumed it was a horse.
Talking Animal: Only to Shin in the special chapters of the manga. To everyone besides Shin, it just sounds like barking. Even outside the special chapters, though, Shin seems to know what Fuji is thinking.