The characters of the Baka and Test series were created by Kenji Inoue, with illustration and character design by Yui Haga. The story centers on a boy named Akihisa Yoshii, also known as the idiot or "baka" of the title. He attends Fumizuki Academy, a school where the staff rigidly divides its students based on the results of their academic scores.

Second-Year Students

Class F The worst class with the worst facilities, including decrepit tables (later changed into upturned cardboard boxes then to large clipboards with rope to hold them up), tatami mats and under-stuffed sitting cushions; 47 males, 2 females, and Hideyoshi make up the class. There is a total of 149 boys enrolled in the second-year class, so approximately one-third are in Class 2-F.

Akihisa Yoshii is the protagonist of the story and acts as the first-person narrator in the novels. He is portrayed as the Ultimate Idiot (Baka among all Baka) and is implied to have the worst grades among his peers of the same level. In the novels, his friends make fun of his idiocy by turning his name, by appending the suffix "-ful", into an English adjective; "He is so Akihisaful" essentially means "He is so stupid" in the context of the novels. Because of his absolute idiocy, he is also given the title of and his Shōkanjū (Summoned Being) is the only Shōkanjū given the ability to touch corporeal objects in order to help the teachers with chores as punishment for being the Ultimate Idiot. Because Shōkanjū are many times stronger than humans, Akihisa can help the teachers perform tasks that would normally require the strength of many people. The flip side to this "special ability" is the "feedback", which means that any damage taken by his Shōkanjū will be transmitted to himself as well (so if the Shōkanjū ("Avatar") was punched in the gut, he would feel the same pain there). Thus, he is always reluctant to fight Shōkanjū battles due to the pain he would receive during battle. However, because he is accustomed to controlling his Shōkanjū, compared to the other students, his Shōkanjū is unmatched in speed, is more agile, and can execute many unorthodox moves (such as sliding attacks), and this has enabled him to win battles otherwise thought to be impossible due to his poor grades. His Shōkanjū's ability to touch corporeal objects is also instrumental in helping his class defeat Class 2-B in Volume 1. Despite being an idiot, Akihisa is generally polite and well mannered among his friends. Yuuji is Akihisa’s closest guy friend and confidant, though they also share a rivalry with one another and tend to play tricks on each other. Akihisa's parents are on long-term overseas work assignments and send money back regularly. However, he always spends this money on the latest games, and as a result, he has to live on merely salt, sugar, and water. In the anime, he is depicted to survive on a sub-divided piece of cup noodles which can be as small as 1/64 of the entire piece. Akihisa has many romantic admirers both female and male, including Mizuki Himeji, Minami Shimada, Hazuki Shimada, Toshimitsu Kubo, and Akira Yoshii (his sister), but is oblivious to this due to his idiocy. Everyone around him is aware of this, as shown when his older sister showed the two girls his fetish and Mizuki tried to attract him by tying her hair, but he still was thickheaded about her feelings. It is also suggested in the anime that both Mizuki and Minami are acting like his girlfriends, unbeknownst to him, and as they do they will try to burn any R18+ magazines he has. A running gag throughout the series is that he is attracted to Hideyoshi Kinoshita due to Hideyoshi's very feminine appearance, going so far as to frequently fawn over him and comment about how hot he is. Despite the fact that they always sabotage each other's reputation, Akihisa has a close friendship with Yūji and can communicate with him just by glances and without any words. Akihisa is also the class ambassador of Class 2-F, appointed by Yūji. Akihisa also has excellent cooking skills, as a result of having to fend for himself because he lives alone, and is good at housework. His favorite dish to make and eat is paella. After the tournament in Volume 2, Akihisa acquired the platinum bracelet that lets him perform dual summons, allowing him to have two Shōkanjū in a battle. Since Akira started living with Akihisa, his grades have seen a significant improvement from what they were before since he has more incentive to study. His Shōkanjū wears a school uniform and wields a bokken. During the ESB field error, his Shōkanjū was unaffected, but it multiplied enough to literally flow out of the school itself.

Mizuki Himeji is the principal female character and one of the smartest students in Fumizuki Academy and would have probably ranked second in her level if not for her inability to finish her placement exam due to her fever. In Fumizuki Academy, leaving in the middle of an exam gets a student zero, regardless of the reason. As a result, Mizuki was placed in the worst class, Class 2-F. Despite the poor conditions of her class which were unfavourable to her fragile health, she claims to be happy because her love interest, Akihisa Yoshii, is in the same class. Mizuki is also portrayed as a shy, timid, innocent, yet ambitious young girl, with a very well-developed bust size (an 'F-Cup' in Japan; equivalent to a 'DD-Cup' in the US). Akihisa and Mizuki used to be in the same elementary school, and since then, she has long admired Akihisa for his thoughtfulness and caring personality. Mizuki also finds Akihisa cute, especially in maid costumes, and was overcome by some sort of weakness when she first saw him wearing a maid costume (when he was chased by Miharu and bumped into Hideyoshi who had some costumes with him). Towards the end of Volume 2, Mizuki changes her way of addressing Akihisa from to . At the end of Volume 8, Mizuki kisses Akihisa to get him to stay home while he is sick, despite Classes F and C having an exam war that day. Even though she has both brains and beauty, Mizuki's cooking is lethal, and in the novel anyone who eats her handmade cooking goes into a coma, including Hideyoshi Kinoshita who is known for his strong stomach. However, she is completely oblivious to the lethality of her own cooking because she does not taste her cooking, and as stated in the novels she adds several chemicals that will create even more potent acids such as the Aqua Regia and hydrochloric acid, also when asked by Yoshii when Yūji, Hideyoshi and Kōta went nearly out cold due to her cooking. She is on particularly good terms with Minami, calling her (by her given name), despite the fact that they are love-rivals. Often, she will emit a killing intent aura and yandere-like eyes whenever she hears that Akihisa is with some other woman, like when Yūji claimed that Akihisa was with a college girl. She will also exhibit extreme denial if she finds any evidence of a girl in his life, such as when she saw his sister's clothes hanging in his apartment—saying that they were "not his size"; to her claiming that a make-up removing cotton puff on his table was a fish cake. This side of her tends to increase more and more as the series goes on. In the anime, it is sometimes suggested that she acts like Akihisa's girlfriend and will try to burn any R18+ magazines he has, along with the aid of Minami. Her Shōkanjū is clad in heavy armor and wields a large sword. During the ESB field error, her Shōkanjū looks like an adult version of herself, while wearing the costume resembling Morrigan Aensland from Darkstalkers. Not to mention that her Shōkanjū's older version is displaying a noticeable exceptional growth in her bust size, which to Mizuki, embarrasses her to no end. Also, in the first theme song, she is holding a six-sided die in mid-air. The kana on its sides spell out "バカとテスト", or "Baka to Tesuto", which is the name of the show.

Minami Shimada is one of the two female students in Class 2-F. She is Japanese but spent most of her life in Germany before coming to Fumizuki Academy, and as a result she is especially weak in kanji. Because of this handicap, the only exam in which she can do well in is math (which still isn't as good as Class 2-A's, due to the presence of word problems). She will also revert to speaking German whenever she's in shock. Minami is the tsundere of the series. Physically, she is slim and has long legs, and can be considered pretty attractive. However, she's somewhat lacking in the 'chest department' (an 'A-Cup' in Japan {she has stated, if they're lifted, they're probably a 'B-Cup'}; equivalent to a 'AA-Cup' in the US {or as she stated, probably an 'A-cup' in US}) and isn't a Valley Girl at all. Due to this, she's often being envious of Mizuki's large bust size, showing an inferiority complex about it (in the anime). Like Mizuki, she too is in love with Akihisa, but due to her inability to express herself and her tsundere nature, she acts violently towards him, often leading him to misunderstand her intentions. During the class battles in Volume 1, while executing a delay tactic, Akihisa indirectly defamed Minami and landed herself in the list of "Top Ten Women Not To Date" among the guys in Fumizuki. In the novel and manga, during a battle in which Akihisa's survival depended on Minami, he was blackmailed to address Minami by her given name, and from then onwards, she started calling him , instead of , while he would refer to her by her given name as well. Whereas, in the anime, this took place when she threatened to break his bones. It is a running gag in the light novel series that whenever Minami expresses genuine concern for Akihisa through her actions or words, Akihisa always draws the conclusion that he is going to be killed or sabotaged by her in some obscure way. Even though they are love-rivals, Minami is on good terms with Mizuki, the only other girl in Class 2-F, and calling her by her given name as well. She's also good at cooking, due to the fact that her parents are not at home most of the time. Like Mizuki (in the anime), it is also suggested that she acts like Akihisa's girlfriend and will try to burn any pornographic magazines that he has. In Episode 7 of Season 2, Akihisa accidentally sends her what appears to be a confession of love through texting, this later causes her to attack him in his sleep, and she ambushes him with a kiss at the end of the credits. Her Shōkanjū is clad in a blue naval military uniform and wields a saber. During the ESB field error, her Shōkanjū looks like an adult version of herself, while wearing a costume similar to Lilith's. Unfortunately, her Shōkanjū's older version still lacks in the bust size, which Akihisa points out (that nothing has really changed).