I was thinking about making an NPC, but I would like to know first, if theres a way to change the avatar, I don't know if this is the name, but it is the actual character, who walks onscreen, and the one that appears on inventory screen.
If its possible, what are the files responsible for these images??

i hope you understand what I am triyng to say, because english is not my language...

Thank you all!!!

Yes there is a way but the avatar (the CH. that walks) and the paperdoll (on the inventory screen) both are tough to change. the avatar is just way too difficult to bother with since you have to have 100's of images to make or edit. this is why not to many have been done.
the pager doll is none too fun either you have to edit plt files match the hands with the weapons.
and most important it all has to be done exactly.

In short, don't bother. Not for an npc. however if you wanted to make an avatar mod in itself then that is fine, just a ton of work
don't get me wrong I do not want to discourage you but help to keep it realistic

welcome to BW

Do you want to make an entirely new avatar or do you just want to change the NPC's avatar to one which already exists in the game? The latter is easy, the former is difficult.