Invisible players

I keep running into invisible players in warfare and have gotten sick of it so I decided to play a vctf game this morning, well, a guy on the other team was invisible, the only way you would ever know where he was at was if he had the flag, when he didn't have the flag he would walk over to our side and just shoot you, you never see it coming. Once you saw him shoot someone else you could just shoot in his general direction. I've downloading the mod for custom characters but don't know where they're at now that I've downloaded them. I've went to the screen where you change your character but don't see anything out of the ordinary. Where are these custom skins people are using that make them invisible, I can't even find the character mods I downloaded off the mod server built into the game.