Korean manhwa about a guy who cannot fight, but ends up winning every fight he is in
Edit: The series is Transfer Student Storm Bringer. 

The girl in this series has pink hair, and the guy has brown hair.



Maybe you are looking for Angel Densetsu.

In the manga, the main character becomes the top fighter of his school due to luck/accident. As a result, many people from other schools come to challenge him to test his abilities, but he wins by luck as always.

He is totally a complete wuss with no experience in fighting, and he is just ugly.



If the manga above is not what you are looking for, try taking a look at the manga listed below. They also have the same trait as Angel Densetsu where the protagonists are just wusses and win by luck.


Ocha ni Gosu
Gorio
Legend of the Strongest Man, Kurosawa
Bari Haken
Transfer Student Storm Bringer (The girl in this series has pink hair, and the guy has brown hair)