Ayumu Kasuga

Unlike the stereotypical Osakan, she seems slow and easily distracted, but she merely has a mind that works quite differently than most people. This makes her prone to daydreaming, absentmindedness, and non-sequiturs, but it also makes her unusually good at answering very specific types of riddles. Her unusual way of thinking gives her a very vivid imagination and a unique way of seeing the world. Osaka often acts stuck in her own world. Although she seems slow and easily distracted, she merely has a mind that works quite differently from that of most people. Though she is often prone to daydreaming, absentmindedness and non-sequiturs, it also makes her unusually good at answering certain types of riddles. While some people assume that she lacks focus, the truth is that in quite a few cases, Osaka is really a little too focused. When she daydreams in class, she tends to dream about Chiyo and her pigtails either controlling her, giving her the ability to fly, keeping her alive, or being replaced with new ones. When she gets a thought in her head, she tends to focus on it so intently that the rest of the world simply disappears, causing her to often miss what's going on around her (an example is in episode 2, when a cockroach invades the class and Tomo is enthusiastically and noisily trying to exterminate the bug, literally sending desks, and presumably students, flying — while Osaka is completely indifferent and only tries to "chase" eye floaters). In episode 6. Osaka kicks her shoe off to predict the weather for the athletic competition the next day, only for it to land on a truck; Osaka and Chiyo's faces tell all. At the same time, Osaka has a habit of being able to point out obvious things nobody else seems to notice (such as the fact that none of her core group of friends has boyfriends or the fact that Kaorin had a crush on Sakaki) and has surprising levels of knowledge on unexpected topics such as marine biology and obscure kanji. Osaka enjoys sata andagi, a type of deep-fried bun similar to a doughnut; In Episode 21 she excitedly and repeatedly says "sata andagi" while holding sata andagi. She also seems to have an obsession with sea slugs; In Chapter 3, she names her team "Team Sea Slug" while in Chapter 33, she plays with sea slugs she finds on the bottom of the ocean. In Episode 15, Osaka enjoys putting her face in flour in the obstacle course, only to lose her chance of winning. When participating in the bread-eating race, her concentration was trounced when the commentator listed the flavour of breads available, causing her to spend too much time deliberating which flavour to pick and, hence, coming in last. Furthermore, she was every bit as interested as the other girls to hear what Minamo Kurosawa had to say while drunk. On their first trip to Chiyo's summer home, Osaka proposed telling a scary story where all of the girls are murdered one by one, only to reveal that in the end, she was the one who killed them all. She also suggested placing cat corpses inside a cafe as an accumulation of the other girls ideas for their second-year Culture Festival, which included a haunted house, a cafe, and stuffed animals. In Episode 25 she dreams about meeting and conversing with Chiyo-chichi while studying in a classroom. In Episode 19, Osaka compares her yawns with that of Chiyo and Tomo.

Osaka hates spicy food and carbonated drinks. According to Tomo, she has a short tongue, and can't hold her breath for more than a few seconds. She is obsessed with splitting chopsticks cleanly and made up a "magic charm" where you got good luck if you managed to do it. Chiyo ended up using the same method when praying for her friends to have good results on their college entrance exams. In contrast to most of her peers, Osaka always keeps a quiet, accessible mood. It's extremely rare to catch her in a fit of anger or losing control (as seen in Episode 25 of the anime, when she sports a maniac grin after finally learning how to break a pair of chopsticks evenly, and gets mad at Tomo for not doing it properly). Osaka is allergic to pollen, like Yomi, and is known for sneezing in an unusually subtle way, going "Ay-tchoh" in the anime and "At-choy" in the manga.

Osaka and Chiyo are close friends, and spend the three years of High School hanging out often. They have a friendly relationship, as the former is Osaka's closest friend from her class. It begins when Osaka joins Chiyo's class and they team up for a P.E. lesson. One of the main ways in which they are similar is that they are both terrible at physical education, as seen when they ran the marathon and both ended up gasping for breath afterwards. They both have kind personalities and are calm in temper. Chiyo is the one who is most intrigued by Osaka's strange theories and thinking and seems to understand them best. This is likely due to Chiyo's childlike naivety which Osaka shares. This aspect of their relationship is perhaps especially clear in the manga. Chiyo also respects Osaka enough to use the formal suffix "-san" when referring to her (as in "Osaka-san"), something she does with everyone except Tomo. Osaka was very sad when she thought that she and Chiyo-chan would be separated because she couldn't find her real name, Kasuga, in the class roster at the start of her second high school year. During the part seen entirely through Chiyo's eyes, she reflects on how often Osaka falls asleep in class and speculates that Osaka might be a night owl. She also appreciated Osaka's efforts in cleaning, albeit she was not very good at it. Right from the beginning of their time spent together, Osaka has been fascinated with Chiyo's pigtails, almost to the point of obsession. She dreams about them several times, daily and nightly. Most of Osaka's dreams in general involve Chiyo in one way or another. In her dreams, Chiyo's pigtails allow her to fly, are detachable, and/or control her. This sometimes scares Osaka so much that she has to test it in real life so as to be sure that it was all a dream. When Chiyo changed her hairstyle, Osaka initially believed that her pigtails had moved on their own. Osaka and Tomo are friends, and spend their three years of High School hanging out often. Tomo was responsible for giving Osaka her nickname. In the second year, Osaka and Kagura join forces with Tomo to create The Knuckleheads. In the third year, Osaka yells at Tomo for incorrectly splitting chopsticks. Osaka later talks about how she and Tomo have got their act together, since they both made it into college. Osaka and Sakaki are friends, and spend their three years of High School hanging out often. She spoke to Sakaki about topics ranging from haemorrhoids to dolphins on the beach during their visit visit to Chyio's summer home. Osaka and Yomi are friends, and spend their three years of High School hanging out often. Osaka and Kagura are friends, and spend two of their three years of High School hanging out often. In the second year, after befriending Osaka and the other girls upon joining Class 3, Osaka joins forces with Kagura and Tomo to create The Knuckleheads. Osaka and Kaorin sat next to each other for the first two years of high school, until she was moved to Kimura's class in the third year where Kagura sat in her place. In the manga, Osaka noticed Kaorin's attraction to Sakaki and asked Kaorin if she was a lesbian, which Kaorin brushed off out of shyness. Osaka was one of Yukari's students during the three years of High School. Yukari mentioned that she had a hard time getting Osaka as a transfer student. Despite her tendency to nod off during lectures, which upsets Yukari, she remains in Yukari's homeroom class throughout high school. Her intention, at Chiyo's advice, to seek a teaching career after graduation (which she later accomplishes) is viewed by Yukari as a deep personal insult, believing that Chiyo thinks teachers are lazy and laid-back. The third time they went to Chiyo's summer home, she was going to wake up Yukari with a "frying pan". She ended up getting a knife by mistake, scaring Yukari, then said: "You woke up by yourself. Then I failed ..." Osaka often attended Nyamo's P.E. classes during the three years of High School, later becoming a P.E. teacher herself after graduating. Osaka disliked Kimura due to his perverted actions towards female students, as did her other friends. Although Kimura remarked in a scene about wanting to see her and the other girls changing, he is hinted in a later scene to be completely disinterested in Osaka, as he went back to class after encountering her in the hallway moments after harassing Kagura.