I'm not sure we ever find out his last name.  Jon is in his late 20s, I would say, and seems initially to be Clara's ardent suitor. At first appearances a sensitive young man, grieving for his lost father and utterly smitten by Clara. He is overjoyed when he finally gets an emotional commitment out of her over dinner, and to celebrate they go to the graveyard to put some flowers on his  father's grave.  This is where Jon shows his true colours.  And none of them mean well for Clara.  Turns out he is a necromancer, a human allied with the dark powers the Vigil is fighting, and endowed by their evil allies with certain magical abilities controlled through intricate tattoos.  Jon has plans for Clara, he has romanced her under false pretenses to get her to this graveyard on this night to kill her and use her "gift" to open a tear in the veil between the realities to get his father back.  Sam will have something to say about that. Needless to say, Jon is not a nice dude. He starts out sounding very "sensitive" and caring, middle section he's the over the top bad magician and the last section, he's defeated and "sealed" - that is to say his powers are taken away, which kind of leaves him sort of lobotomized.  He is a recurring character, he will  be in a couple more issues, His parts in later issues aren't huge, but are very important.