To unlock Wolf, you must've first completed Subspace Emissary. In Subspace Emissary choose to go to "The Ruins" and choose any difficulty and any characters to go. When in the stage keep going as you would in any Subspace Emissary stage and the first door you see you don't have to enter unless you want trophy (not a specific trophy) and a Heart Container which you most likely won't need since the next door you go to, which is very nearby, will be where Star Wolf is and when starting the battle with him you will start at 0% damage. When entering the second door get ready to fight Wolf, and again don't worry if your damaged, you'll start all over at 0%. After defeating Wolf, you'll unlock him.