Waterboy
Waterboys are men or boys who willingly run errands for girls that they are interested in, however the girls are not interested in them. They do things such as taking her trash and disposing it. Buying and bringing her food. Getting her water. Whether she asks you or not.

One key difference between a guy 'just being nice' and a guy who is a 'waterboy' is that the waterboy does this specifically for one or more girls, and does not do these same actions for his male colleagues.

Girls usually pick up on this and will use waterboys to run mundane errands.
"Wow, that guy who's been flirting me keeps cleaning up my space and buying me food. I guess I have a waterboy now."

"Dude, you should stop doing things for her, your just being her waterboy. She's using you, bro."