Wolf is a member of Group 13. He is played by "Blazing Waffles".
Very little is known about Wolf's past, but what is known is that his father (currently deceased, killed by Wolf himself) has something to do with the Infection, and his mother is dead. After weeks of wandering on his own, Wolf was approached by the legendary Group 13, who offered him a place among them; he refused. Days later, in a chance meeting with a thief, Wolf's life was saved by Colton. He joined Group 13 soon after.
In the months that followed, Wolf assisted the team in killing a large number of Infected, and bringing in survivors. After the plane crash into Chicago, Wolf found his way to Colton's location, where he also met Jamie. He killed a Banshee after it murdered Kirkland. Soon after, he was half-infected by breathing an airborne version of the virus. He began to lose his sanity, which led to a number of enemies - human, infected, and...not.
Wolf has been regarded as the most aggressive and brutal of Group 13, a trait he learned from weeks of isolation and terror. Most of the more violent myths about Group 13 were from his exploits, though greatly exaggerated (usually). He's a bit of a sadist, and genuinely enjoys killing Infected more than helping others, even if cause-and-effect is in play. On the other hand, he's intensely loyal to those he trusts - he would lay down his life for the rest of his team. Beyond that he's a loner, very cynical and sarcastic. He has a gritty outlook on life, which isn't helped by the stark reality of the situation.
Post-infection, Wolf's violent tendancies grew ten-fold; combined with his enhanced strength and speed, he's a literal killing machine. Also of note, he's slowly slipping down the slope to full infection - at which point he would have to be euthanized by the rest of Group 13. His infection is also the cause of his visions, dreams, and messages; the ghost girl he sees is likely a hallucination brought on by the mental trauma, but not everything adds up.
Another phenominon would be the third voice in his head, beside the girl's; a woman seems to be warning him, telling him to "come home". Who (what?) this woman is hasn't been uncovered.