This poor kid is the object of Saskia's demented toying.  Because of this, he has been diagnosed with several disorders only a few of which he actually has and the rest of them being randomly inflicted upon him by Saskia. 

In high school he's the object of ridicule and rumors due to the weird behavior he has (inflicted by Saskia). Kito's his only friend until jr. year where he gains Avira as a friend as well. Through these years he's scrawny weakling.

In adulthood Johnny has figured out the world of demons and has become a demon hunter in order that no one else falls prey to one like he did. He quickly escalates to a formidable fighter as hunting becomes his obsession.