School Rumble 2-C

Tani - 2-C's homeroom teacher. He teaches english.
Asou Hiroyoshi - First appeared in volume 1 during the pool fight. Frequently occuring character.
Hiroaki Ushiyama - Appears mostly in the background, was most visible during the teahouse vs theater battle, played mahjong with Saitou, Suga, and Tougou while talking about girls in b39
Kyosuke Imadori - Frequently occuring character
Shigeo Umezu - First appeared in a b chapter in vol 5. Appears sporadically but is easily identifiably. Usually not seen in background
Ooji Karasuma - Main character but very infrequent appearances in recent volumes
Ryuhei Suga - Mostly known as Asou's sidekick. Has gotten a lot of attention in some b-chapters. Known for his inability to get girls
Kazuya Tanaka - Appears mostly in the background. Had a b-chapter that starred him and Sara. Dating Toki Nagayama
Kentarou Nara - Debuted in the first beach story. Tried to date Tenma. Now he's stuck being one of the three losers of 2-C (Nara, Yoshidayama, Nishimoto)
Nishimoto Ganji - One of the first background students. Was the guy with bad eyesight who switched with Tenma. When he reappeared, he was given the reputation of being a "ero sommelier" (aka big porn collector). Had a chapter where he gives his porn away.
Haruki Hanai - Seen in the background in volume 1, debuted at end of volume, main character
Kenji Harima - Main character. Appears more than anyone else. Most chapters are focused on him.
Takeichi Fuyuki - Appeared early on as a boy who takes voyeur photos. Seems to be friends with Tenma's clique and is part of the band clique (Fuyuki, Ichijou, Sagano, Yumugi, Ooji)
Shin Misawa - A background character with almost no appearances. His last name is mentioned during the Teahouse - Theater battle. He was also the third boy holding up Mikoto during the horse battle (the other two were Asou and Suga)
Jirou Yoshidayama - First appeared misinforming Harima of his class in Harima's debut. Reappeared as Harima's "rival". Now is just a part of the three losers.
Saitou - Frequently appears in the background, he finally debuted in chapter b-39 where his crush on Ayano is revealed. In 186, he tries to hit on her but fails.
Unknown Boy 1 - Looks like Saitou but he has bleached hair. Appears in the background only, has some dialogue. Not in the anime. Chance of being named in the future: 50%
Unknown Boy 2 - Probably most noticeable background unnamed boy. Debuted as the third boy holding up Eri during the horse battle (other two were Harima and Yoshidayama). Was quite visible during the theater - tea house battle, and even had the sideline mention him as a brat. He was in the 3rd character popularity contest under the name "2-C Boy 2"
Unknown Boy 3 - Like Unknown Boy 1, this guy appears in the background, although quite rarely. He has a few lines of dialogue. Chances of being named in the future: 10%
Unknown Boy 4 - He helped carry Tenma during the horse battle. It's quite possible that he has never said anything. Shows up in the background only. Chances of being named in the future: 5%

Karen Ichijou - Debuted in vol 4. Frequently occuring character.
Mai Ootsuka - First appeared in the background during the pool battle, hanging out with a girl that is either Saeko or Madoka, and Rinko. Later debuted as the female class rep. Considered a frequently occuring character.
Kido Madoka - May have first appeared in the background during the pool battle (hard to tell between her and Saeko). Debuted with Umezu in vol 5's b chapter. Doesn't really show up often but when she does, the story usually focuses on her or mentions her.
Megumi Sagano - Frequently appeared in the background until she debuted as part of the band clique. Featured in b-chapters. Friend of Tenma. Starting to show up more and more. Nicknamed Saganon.
Eri Sawachika - Member of Tenma's Clique (Tenma, Mikoto, Eri, Akira). Frequently appears.
Mikoto Suou - Member of Tenma's Clique (Tenma, Mikoto, Eri, Akira). Frequently appears.
Akira Takano - Member of Tenma's Clique (Tenma, Mikoto, Eri, Akira). Frequently appears.
Tenma Tsukamoto - Main Character.
Toki Nagayama - Appeared in the background mostly. Was an unknown girl until she was featured in the Teahouse - Theater battle. Asked out by Tanaka. Author Jin Kobayashi supposedly called her a "hidden treasure", perhaps refering to her background appearances.
Kozue Mihara - First seen chatting with Imadori. Part of the girls' relay race. Was on the theater army. Appears very infrequently and has not been featured in a story, but is quite visible because of her kogal appearance.
Yuuki Tsumugi - Mai Ootsuka's sidekick. Appears in the background a lot. Had a b-chapter devoted to her which revealed her crush on Hanai. Considered a frequently occuring character.
Mina Yukino - First appeared during the baseball chapter. Frequently appears in the background with Rinko. Had her most screentime during the Theater - Teahouse battle. Interesting note: Her last name was seen in the tea house list of girls but it wasn't until the anime that the name was associated with her. Mina was in the 3rd character popularity contest undert he name "2-C Girl 2"
Saeko - May have appeared during the pool battle but that could have been Madoka. Appeared very infrequently. Got her most screentime during the culture festival where she catered to Goriyama. She actually appears less than the unknown girls!
Unknown Girl 1 - Has some moderate appearances in the background. Member of the teahouse army. Talked to Tenma in the restroom during the culture festival. Apparently appeared on vol 12's cover insert.
Unknown Girl 2 - Has some moderate appearances in the background. Apparently appeared on vol 12's cover insert.
Kaori - Very infrequent appearances, but she's there, mainly in the early volumes. Her name is revealed on the roster of girls in the tea house but she is never actually called Kaori by anyone. She is also quite tall compared to the other girls.
Ayano Kinugawa - Originally appeared in the chapter about Otoko posing nude. Was prominent during the teahouse-theater battle arc. In the anime, one of the unknown girls was replaced by Ayano in the bathroom scene with Tenma. Was part of the play. Her family owns a bathhouse. Saitou has a crush on her. Appears to be shy as she doesn't speak much.
Rinko - Possibly the most frequently appearing background character, named or unnamed. Has been around since Harima's debut. Most prominent appearance was during the play where she was the narrator. 2ch fans called her "Rinko" (as in Neighbor Girl) because she sat next to Harima and the anime producers decided to name her as such. Technically unnamed in the manga although it is likely that Jin Kobayashi will go ahead and call her Rinko. Rinko was in the 3rd character popularity contest under the name "2-C Girl 1"

Note: When Tenma looks at the 2-C roster in the first chapter, the name Tonami Junko appears below it. Also in a fairly recent chapter, Saitou mentions the name Fujikou as a girl who went out with Saeko and Madoka. These two names may possibly be the names of the two unknown girls, although we have no idea which is which.