At the Five flagons Inn,.. there's a female warrior to the right of the bar named Belle. She's allways making comments like:
" mess with me and you'll regret it" .
Who is she ? Is she quest related somehow ?

ive talked her quite a few times and i dont think she is quest related

If memory serves me correctly, use Mazzy to talk to the barkeeper, then talk to Belle. The barkeeper will give you something, and Belle will just have some friendly banter with Mazzy.

If it has anything to do with the subject, Belle was the "escort service's" sheep in Arcanum. Maybe got shapeshifted and bitter, sort of like Edwin.

dundee is right, Belle has something to do with mazzy. she is like mazzy's company... something like bounty hunters. talk to her then the bartender will give you some sorth of valuable things where you can sell in exchange of money.