Seems that way, but I just can't believe it's really him. I think it's a red herring and there's at least 3 stalkers (Zak, Johno and someone else [maybe the detective?]). Zak must know that houses like Charlie's have alarms, cameras, etc. and even if he won't get caught (meaning he should be really good at entering private properties, smth I don't believe he spent a lot of time to learn) he should know it's a potentially dangerous action and a felony; meaning it's too great a risk to take if his only desire is to have sex with Charlie and then move on to new victims (also leaving photos, messages and sperm doesn't help him in any way, so it wouldn't make any sense for him to do these things [and there's no reason for us to believe he's some kind of psycho]). Also why would he masturbate with Emma's picture if he finds her unattractive? Plus he doesn't seem to be a type of person who masturbates to the photographs imo, he likes to "collect" girls that he finds attractive/popular, he doesn't feel any other kind of attraction towards them; and masturbating to the photo [as it's presented in the game] suggests masturbating person's obsession towards the person on the photo. Of course, I could be mistaken, but atm I don't think that Zak is the stalker (also he would make for a lame adversary for MC tbh, and I don't think the dev would do smth like that). Though I don't know when and how he was in Charlie's house (yet).