Blue

The effect of trauma on children with the mutant gene is a severely underfunded area of research. There are suspected to be thousands of cases across the United States, but it is very poorly understood.
Blue is a trauma survivor that was not born with an active mutant gene, but developed powers later in life to protect himself. He suffers from PTSD along with other characteristic trauma related conditions.
He is an enigma to the X-men and Professor X for a variety of reasons, the most interesting being that he does not show up on Cerebro, as if he didn't exist at all. However, his deteriorating mental health was their primary concern.
When an attack on the mansion leaves Blue MIA, the X-men are forced to track him down the old fashioned way. They follow a string of curious murders, working to find him and save him before he loses himself for good.