Ayumu Kasuga is a transfer student from Osaka, Japan. Tomo Takino expects her to act like the stereotypical Osakan — loud, like a hillbilly, chose for her the nickname "Osaka", although her behavior could not be more different. She reminisces how people in Osaka consider her "slow" and "spacey", at least compared to the average Osakan, but she lacks the energy or motivation to change. Ayumu was the first to ever lose a race against Chiyo Mihama. The nickname sticks so well she is called 'Osaka' by teachers and even in the class roster, to the point that no one even remembers her real name (Yomi has a similar problem). In the manga, Osaka mentions that she lived in Kobe when she was in elementary school and was born in Wakayama.
Despite her tendency to nod off during lectures, which upsets Yukari, she remains in Yukari's homeroom class throughout High School. Because of her poor performance on exams, she is a member of the "Knuckleheads" ("Bonklers" in Japanese, "Numbnuts" in the English manga and "Boneheads" in the Yen Press translation), together with Tomo and Kagura. Her intention, at Chiyo's advice, to seek a teaching career after graduation is viewed by Yukari as a deep personal insult, believing that Chiyo-chan thinks teachers are lazy and laid-back.
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). She is especially close, and similar, to Chiyo, since even though Chiyo gets much better grades, both are bad at P.E., and both seem to approach the world with the same sense of childlike wonder and curiosity, though Chiyo's is usually tempered by common sense and information, while Osaka is not. Chiyo provides this for her, habitually reaching to grab Osaka's shirt sleeve when Osaka begins to muse about doing potentially dangerous things, and presumably might follow through if not restrained and "brought back to earth" (like in an episode of the anime, when eating lunch on the school rooftop, Osaka wanted to jump off the top and try to "fly").
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).
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. She also seems to have a strange obsession with sea slugs. Furthermore, she was every bit as interested as the other girls to hear what Nyamo 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. Also, the 3rd time they went to Chiyo's summer home, she was going to wake up Yukari-sensei with a "frying pan". She ended up getting a knife by mistake, scaring Yukari-sensei, then said: "You woke up by yourself. Then I failed ..."
During the Sports Festival, she made a big deal out of running the obstacle course because she thought the event of sticking her face in flour to get candy was funny and had a giggle fit when she got there (and lost). Afterwards, her face was still covered in flour. She has almost no flexibility in her body, unlike some of her other friends. The manga also revealed that she actually likes to run outside and play in typhoons, yet is easily spooked by thunder.