Takatoshi Tsuda (津田 タカトシ Tsuda Takatoshi?) Voiced by: Shintaro Asanuma
The main character of the story, he chooses to attend a former all-girl school simply because it is close to his home. On the first day at the school, he is forced to join the student council as vice president and male representative. He normally acts as a tsukkomi to Shino and Aria's perverted innuendo and comments, and becomes so used to it that he would actually feel odd if they did not make such jokes.
Shino Amakusa (天草 シノ Amakusa Shino?) Voiced by: Yōko Hikasa.
The student council president and a second-year student. She is serious and diligent, talented in most subjects, and is also very popular among the student body. However, she is almost always thinking of perverted things, even suggesting that one of her main reasons to be interested in Tsuda is curiosity about information not covered in health classes. She fears heights and insects, and has a complex about her chest, which is exacerbated by the fact that Aria happens to be better endowed than she is. She later develops some feelings for Takatoshi.
Aria Shichijō (七条 アリア Shichijō Aria) Voiced by: Satomi Satō
A second-year student who is the student council secretary and Shino's best friend. She is the most mature character in appearance, and comes from an affluent family. However, her mind tends to follow the same paths as Shino, possibly more so, and seems especially interested in bondage and exhibitionism. She tends to be airheaded at times, the consequence of being absurdly rich and overly pampered. Being more well-endowed than her fellow student council members (and apparently still growing) exacerbates Shino's complex about her chest.
Suzu Hagimura (萩村 スズ Hagimura Suzu) Voiced by: Sayuri Yahagi
A first-year, 16-year-old (17 later in the manga) genius return student with an IQ of 180. She is the student council treasurer and the most mentally mature of the characters, but she is very sensitive about her body, which still appears to be that of an elementary school student. Many of the jokes involve Suzu's focus on her childish appearance or lack of height, which cause her to fly into a rage whenever it is mentioned. Being both first-years and the more sane members of the group, she develops a rather close (platonic) relationship with Takatoshi (to the point that they are able to communicate using nothing but facial expressions). Later on, she develops romantic feelings towards Takatoshi.
Kotomi Tsuda (津田 コトミ Tsuda Kotomi?) Voiced by: Asami Shimoda
Takatoshi's younger sister, who ultimately also ends up attending Ōsai because of its proximity to home. She is considerate and caring toward others, but is curious and enthusiastic about adult matters as well, to the point of frequently behaving similarly to Shino (both brought Takatoshi adult manga when he was sick). She likes to embarrass her big brother by making comments that suggest they are physically involved. she is friends with Kaori, trying to get her to "lighten up" and obey the rules.
Ranko Hata (畑 ランコ Hata Ranko?) Voiced by: Satomi Arai
The head of the school's newspaper club, who has a habit of taking photos of the student council members to sell around the school, usually without the knowledge of those being photographed and occasionally faked. She often tends to get caught. She is given a dead-pan, monotone voice in the anime, which strongly contrasts with the other girls' excitable temperaments.
Naruko Yokoshima (横島 ナルコ Yokoshima Naruko?) Voiced by: Yuu Kobayashi
A teacher at Ōsai Academy, who happens to be the advisor of the student council. She is incredibly perverted, even more so than Aria and Shino, and aggressively seeks out younger guys, including her male students. She is often portrayed as unreliable and useless from the point of view of an educator.
Mutsumi Mitsuba (三葉 ムツミ Mitsuba Mutsumi) Voiced by: Chiaki Omigawa
The leader of the school's judo club, and Takatoshi's classmate. She is a very strong and skilled fighter, and very dedicated to leading her club, but is also simple-minded. She develops a rather obvious crush on Takatoshi and, despite her remarkable potential and drive towards the martial arts, is occasionally paralyzed when she comes into (physical) contact with him. She is kind and innocent, often misinterpreting the perverted comments of Aria and Shino. Her ultimate dream is to become a bride.
Chiri Nakazato (中里 チリ Nakazato Chiri?) Voiced by: Yukika Teramoto
A tomboyish friend of Mutsumi who is one of the original founders of the Judo Club.
Kenji Yanagimoto (柳本ケンジ Yanagimoto Kenji?) Voiced by: Minoru Shiraishi
Katatoshi's male classmate. He is one of the few male students in Ousai Academy except Takatoshi that made recurring appearance and even has spoken lines.
Kaede Igarashi (五十嵐 カエデ Igarashi Kaede?) Voiced by: Emiri Katou
A freshmen at Ōsai Academy, she is the president of the disciplinary committee, who has a strong sense of justice and morals. However, she has an extreme fear of men(androphobia), when she is near/touches a male she will freeze up and go silent and stiff. By the second season she is able to be around Takatoshi (as long as he does not touch her).
Sayaka Dejima (出島 サヤカ Dejima Sayaka) Voiced by: Mutsumi Tamura
Aria's personal maid, extremely perverted, a skilled chef who particularly enjoys doing the laundry. She is very protective of Aria, being the keeper of the key to her chastity belt, and seems to have a fetish for anything that Aria has touched or worn, as well as unwashed underwear in general.
Nene Todoroki (轟 ネネ Todoroki Nene) Voiced by: Hekiru Shiina
A member of the Robot Research Club who loves to build both complex robots and vibrating sex toys, usually wearing one throughout the day. She is a personal friend of Suzu.
Kaoru Toki (時カオル Toki Kaoru) Voiced by: Miho Hino
Kotomi's first friend she made since entering high school, who looks and sounds like a delinquent. Despite her scary and anti-social appearance, she is incredibly clumsy and shows no signs of a rebellious nature; in fact, the only reason she leaves her shirt loose was because she accidentally tucked it into her underwear one day and was embarrassed about it. She also has a habit of wearing jackets inside out (and not realizing it.)
Chihiro Uomi (魚身チヒロ Uomi Chihiro) Voiced by: Chiwa Saitō
The president of Eiryou High School, Uomi first meets the members of the Student Council during a school mixer. She hits it off with Shino (due to similar personalities) and becomes a regular visitor. She later becomes Takatoshi's relative through marriage (her cousin and his cousin got married) and becomes a regular visitor to his house (comfortable enough to shower and leave some of her clothes there).
Nozomi Mori (森ノゾミ Mori Nozomi) Voiced by: Sumire Uesaka
The vice-president of Eiryou High School, she appeared for the first time when Shino and Uomi swamp school for a day. A polite-talking girl, she must endure the perverted ways of her President, making her an Eiryou equivalent of Takatoshi (often described for Uomi as Eiryou's Tsukkomi).
Misaki Amano (天野 ミサキ Amano Misaki) Voiced by: Yukika Teramoto
A younger childhood friend of Shino who is very familiar with her twisted way of thinking from their past in an elementary school organization, where Shino was also president. Misaki seems about as level-headed as Takatoshi. She is actually a character from another of the author's manga.