The main characters are not very likable. Banba is probably the most likable character, and he's just a supporting senpai.
Hanza is...well, he doesn't really inspire animosity, but he doesn't have very many admirable traits either. He's just extremely determined and full of athletic potential, which are basically shounen staples. It's sort of annoying that what drives him to try to leave his deliquent past behind is nothing more than hormones. As the norm in manga, the parents/family are nonentities, so they're not factored in his decisions.
And yeah, he has talent for karate, but how his actions in the past affected those around him is never even addressed. That's all in the past (as in excised from character history) because he's crushing on a girl and going to learn organized combat skills to impress her.
He is relentlessly persistent, to be redundant, and I guess that's a positive for shounen heroes. Nothing ever seems to "get him down", in all manners of speaking.
The female lead character is unlikable. Which is disappointing especially because she had such potential as an exceptionally physically strong female lead, with combat skills, a strong sense of justice, and a backbone. Unfortunately, this is all poisoned by her constantly jumping to conclusions, refusing to listen to what others are saying, and sticking to her monochromatic views on Right/Wrong despite her erroneous judgments. She is egotistical, self-righteous, arrogant, myopic, bull-headed, and not very bright.
I'm going to be honest and say that if she were a male senpai, and Hanza was not cushing on her, her actions wouldn't be out of the norm. She'd be a Strict Senpai (who learns the MC isn't that bad) if she were male, and she wouldn't be hated so much by readers.
It's a shame.
The series as a fighting manga does get better from about chapter 17, when the actual matches begin and the mangaka stops emphasizing Hanza's crush on Fujiki, but before that...it's all a bit "eh".
I'd rec Full Contact and Green Boy(manhwa) as better "boy gets stronger mainly to impress girl" martial arts series, but neither have particularly strong/outspoken/more-than-a-trophy female leads.
This series does have decent art, though. Significantly better than Full Contact.
All Rounder Meguru does the whole Female Lead who Fights thing much, much, much better, though. Even History's Greatest Disciple with it's ridiculous body proportions and ecchi does a better job in that field.
All in all, not the worst I've read, but very far (miles away) from being the best.
Edit: story goes from being 3 or 4/10 to being a decent 6 or 7 once the mangaka drops the "romance", and focuses on karate, the team, and character growth.