may contain some errors, since i haven't heard all the names spoken yet. question marks are for given names that can be read different ways, but i used the most common/most likely reading. Last names are taken from stations/areas in the Tokyo area (with the exception of Risa's character), so they are most likely correct.
Main: main character in an episode (pretty much just Niina, since I did not list the lead male role of Kazuki).
Lead Story: principle character in the story arc of the episode.
Story: supporting character in the story arc of the episode.
Lead Supporting: heavily featured character in the episode (but may not have been part of/significant to the story).
Supporting: supporting character featured in the episode (had lines, but may not have been part of/significant to the story).
Morning Musume
Toriumi Nagisa (鳥海 渚) - Niigaki Risa (新垣里沙)
Episodes: 1-4 (supporting)
School nursle that sometimes gives Niina (Reina) advice, and knew her sister. Has a fish tank in the courtyard for some reason.
Tachikawa Sayuri (立川 さゆり) - Michishige Sayumi (道重さゆみ)
Episodes: 1, 4 (lead story), 2-3 (lead supporting)
Initially an antagonist, now fangirls Niina (Reina) out of admiration for her math skills and has since become a part of Niina's group. Is self-centered and can be malicious to people she doesn't respect or like, but clingy and indulgent to the people she likes or admires.
Machida Niina (町田 ニーナ) - Tanaka Reina (田中れいな)
Episodes: 1-4 (main)
Main character. She took an interest in Kazuki (the lead male character), though she treats him as her "henchman". Is trying to become the math boss to find out what happened to her sister (who was previously a math boss before her disappearance). Is bossy and spoiled but always helps out her friends when they're in trouble, albeit sometimes for selfish reasons.
Akasaka Haru (赤坂 春) - Fukumura Mizuki (譜久村聖)
Episodes: 2 (supporting), 3 (story)
Part of the glutton girls group lead by Fuyumi (Maasa).
Nakano Riko? (中野 理子) - Ikuta Erina (生田衣梨奈)
Episodes: 2 (supporting), 3 (story)
Part of the glutton girls group lead by Fuyumi (Maasa).
Nagata Sana (永田 沙奈) - Sayashi Riho (鞘師里保)
Episodes: 1-3 (supporting)
Serves as referee & MC for the math battles.
Ootsuka Mei (大塚 メイ) - Suzuki Kanon (鈴木香音)
Episodes: 2 (supporting), 3 (story)
Part of the glutton girls group lead by Fuyumi (Maasa).
Udagawa Rin (宇田川 りん) - Iikubo Haruna (飯窪春菜)
Episodes: 2 (supporting)
Part of a patrol squad lead by Mami (Maimi) who march through the halls and report to her about any disturbances.
Jinnan Runa? (神南 瑠菜) - Ishida Ayumi (石田亜佑美)
Episodes: 2 (supporting)
Part of a patrol squad lead by Mami (Maimi) who march through the halls and report to her about any disturbances.
Miyamasu Rion (宮益 りおん) - Satou Masaki (佐藤優樹)
Episodes: 2 (supporting)
Part of a patrol squad lead by Mami (Maimi) who march through the halls and report to her about any disturbances.
Miyashita Noa (宮下 のあ) - Kudou Haruka (工藤遥)
Episodes: 2 (supporting)
Part of a patrol squad lead by Mami (Maimi) who march through the halls and report to her about any disturbances.
Berryz Koubou
Azabu Miku (麻布 未来) - Shimizu Saki (清水佐紀)
Episodes: 4 (supporting)
Part of the Morality Squad led by Iroha (Yuu). Is very loyal to her squad leader.
Harajuku Tomoko (原宿 ともこ) - Tsugunaga Momoko (嗣永桃子)
Jinguu Takako (神宮 貴子) - Tokunaga Chinami (徳永千奈美)
Episodes: 4 (supporting)
Part of the Morality Squad led by Iroha (Yuu). Is very loyal to her squad leader.
Ookubo Fuyumi (大久保 冬美) - Sudou Maasa (須藤茉麻)
Episodes: 3 (lead story)
Leader of the glutton girls, and is a mother figure to them. Is the most skilled in finding the fastest way to the lunch room.
℃-ute
Shibuya Mami (渋谷 真実) - Yajima Maimi (矢島舞美)
Episodes: 2 (lead story), 3-4 (supporting)
Aspiring police detective; she gets excited when mysteries present themselves that she can attempt to solve. Has a squad of four girls (played by Haruka, Haruna, Ayumi and Masaki) who patrol the school halls and report to her. Trivia: first name can be read as the word "shinjitsu", meaning truth or reality.
Uehara Yuuri? (上原 優梨) - Suzuki Airi (鈴木愛理)
Episodes: 5 (lead story)
Komaba Satoko (駒場 聡子) - Nakajima Saki (中島早貴)
Mita Tomoko? (三田 智子) - Okai Chisato (岡井千聖)
Yotsuya Yuuko? (四谷 優子) - Hagiwara Mai (萩原舞)
Soloists
Shirogane Reiko? (白金 麗子) - Mano Erina (真野恵里菜)
Aoyama Iroha (青山 彩葉) - Kikkawa Yuu (吉川友)
Episodes: 4 (lead story)
Leader of the Morality Squad, a group of three students including Miku (S. Saki) and Tomoko (Chinami), that enforces school rules and seeks to punish rule-breakers. She is distrustful of people's morality and loyalty, without realizing that her own squad are very loyal friends to her.
S/mileage
Tengenji Sora (天現寺 空) - Wada Ayaka (和田彩花)
Episodes: 1-4 (supporting)
Part of an energetic group with two other girls, Umi (F. Kanon) and (Meimi), whose purpose seems mainly to gossip and provide comic relief. Trivia: name means "sky".
Yamashita Umi (山下 海) - Fukuda Kanon (福田花音)
Episodes: 1-4 (supporting)
Part of an energetic group with two other girls, (Ayaka) and (Meimi), whose purpose seems mainly to gossip and provide comic relief. Trivia: name means "sea".
Ookura Riku (大倉 陸) - Tamura Meimi (田村芽実)
Episodes: 1-4 (supporting)
Part of an energetic group with two other girls, (Ayaka) and Umi (F. Kanon), whose purpose seems mainly to gossip and provide comic relief.
Hello Pro Egg
Meguro Yuu (目黒 ゆう) - Miyamoto Karin (宮本佳林)
Daiba Aki (台場 あき) - Tanabe Nanami (田辺奈菜美)
Trivia: name means "autumn".
Maihama Haru (舞浜 はる) - Takagi Sayuki (高木紗友希)
Trivia: name means "spring".
Hakkeijima Natsu (八景島 なつ) - Yoshihashi Kurumi (吉橋くるみ)
Trivia: name means "summer".
Up Up Girls (仮)
Hatsudai Mina (初台 みな) - Saho Akari (佐保明梨)