Ryuuichi Kurosaki (黒崎 龍一 Kurosaki Ryūichi) is a delinquent guy whom Yoshiko Hanabatake interacts with and he becomes her disciple, calling her anego (アネゴ, "big sister").Ch. 4, 6 He tries to become friends with Akuru Akutsu but fails because of his shyness, and ends up acting more like his underling.Ch. 29 While he may seem stupid, Ryuuichi has proven himself to be very smart as he once stayed home and studied even placing higher than Akuru who was shocked that Ryuuichi was skilled in academics.

When Ryuuichi first appeared, he was shown to be just your typical delinquent. However, after meeting Yoshiko Hanabatake and Akuru Akutsu, he became a lot nicer and shows his idiotic side more.

Yoshiko Hanabatake is an idiot beyond all belief. Somehow managing to consistently score zeroes on all of her tests and consumed by an absurd obsession with bananas, her senseless acts have caused even her own mother to lose all hope. Only one person is up to the task of keeping her insanity in check: childhood friend Akuru "A-kun" Akutsu.
 
Though he bemoans the ridiculous behavior he has to endure, the studious but terrifying A-kun is always ready to put an end to any stupidity Yoshiko gets up to, with no qualms about using physical force. Unfortunately, no matter how many times he attempts to knock some sense into her, the girl bounces right back to her usual shenanigans, even dragging in some other eccentrics along for the ride. Try as he might to rein in her nonsense, every moment is unpredictable with Yoshiko and her profound idiocy on the loose.
 
(Source: MAL Rewrite)

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Yoshiko is a hopelessly clueless high school girl. Her rock-paper-scissors strategy is “rock.” She somehow gets zeroes on multiple-choice tests. And she has an often-nearly-fatal weakness for bananas. Her devoted friends have no choice but to hang around and make sure nobody takes advantage of her. (Of course, the entertainment value might have something to do with it, too.) Cute characters, weird gags, and lots and lots of bananas!
 
(Source: Kodansha USA)
 
Included one-shot: 
Volume 12: Nandemo Shiteageru (I'd do anything for you.)