Kiseijuu - guy starts out a little wimpy, becomes badass. Romance isn't all that prominent but it's definitely not "subtle" either in the way you mention.
Midori no Hibi - decent romantic comedy with a delinquent main character
Angel Densetsu - hilarious manga, with a strong protagonist that may or may not be "badass" in a way you appreciate... there's enough romance to keep one interested, and while it not may be as explicit as you might want... they're pretty much an established couple for a significant portion of the manga.