the brown haired protagonist is essentially the build of a young adult. he's definitely in his teens, or he's a dwarf.
but Zell knows quite well that people don't play the game for the protagonist, I think that's why he put Vore in the game, to let the protagonist know his place = "the players are only in it for the ladies.", that and you can clearly tell like with the Matango during sex the guy fades out completely so he's seethrough. because Zell knows not to cock-block, thats just not his vision for the game..
Further look at the design of the protagonist; brown clothes, in a basic design, brown hair in a basic cut and style, basic face. If I were to guess this was intended to be like a "default guy mightaswellnotevenhaveaface" situation.
He may be important to the story, I've noticed some interesting things; (potential spoiler though not sure)
the bench fox-girl has the same hair color, and size as him. After time seeing how they "cooperate" and she's at his home base; the kinda story I get here is the protagonist is the bench girl's brother. They were adventuring and travelling and found their way into the Forest of the blue skin, something then happened to his sister changing her into a kitsune of some form, and he set off deeper into the area to rescue her and turn her back to normal. She is always placed at the spot where new passages will be placed, as if guiding him through the land to the one(s) who turned her into a monstergirl. at least this is my interpretation of the story. (there is no official story as of yet)