At first I thought it was “Windbreaker” but when I went back to re-read it, I realized this wasn’t the right manga I was looking for. Anyways all I remember is that there was this girl who called out another girl to the gym and then slapped her (basically trying to fight her) because she was jealous the ML was giving her attention. Then the ML appeared and saved the girl, but he ended up dating the girl who was the instigator (i think). I think he was really weak at first too but then he started working out and got buff and that’s when the mean girl started falling in love with him.
I just can’t with the stupidity of these characters. The ML is the worst/trashiest man ever and FL dumb as hell.