Shijou Saikyou no Deshi Kenichi

Whenever he is not getting pummeled in the karate club after school, high schooler Kenichi Shirahama can be found with his head buried in one of the many self-help books he carries with him every day. One day, on the way to school, Kenichi accidentally frightens the new student in his class, leading her to throw him down onto the ground. After apologizing for her violent reflexes, she introduces herself as Miu Furinji and asks Kenichi to be her friend. Later that day, after karate club practice, Kenichi is given an ultimatum by one of the other club members: they will fight next week and the loser has to quit the club permanently. While lamenting his imminent defeat in the upcoming fight, Kenichi witnesses Miu fend off three thugs easily. Fed up with being perpetually bullied by those more powerful than him, he asks Miu for tips on how to become a better fighter. She directs Kenichi to her grandfather's dojo, Ryouzanpaku, where several skilled—albeit strange—masters of extreme martial arts have gathered to hone their abilities. Miu promises that Kenichi will become significantly stronger, but only if he manages to stay alive. With those ominous words looming in his mind and the appearance of a criminal martial arts gang known as "Ragnarok" threatening the safety of those around him, Kenichi takes the first step on his journey to become a world-class fighter and a defender of the defenseless.

Plot summary from MyAnimeList. Data and images via Jikan API.