It seems like this playbook's underlying socially awkward teenage issue is a feeling of fundamental inadequacy. This character is constantly trying to prove that he's good enough, because he's so afraid that he's not good enough.
The stats are interesting, in that they assure that the areas in which you excel are different from the rest of the group, which makes sense. However, I am inclined to say that Xander (not that this playbook has to literally be Xander of course) himself was Volatile (always running away from trouble or right into it) and Hot ('cause for the awkward guy, everyone wanted him), and he sure as heck wasn't Dark or Cold.
This version of the Mortal's sex move is extremely interesting, because it means that this Mortal goes out of their way to help their friends in their darkest moments. If you have sex with someone in their Darkest Self, they're free of it sure; good luck getting there with your skin intact, when you're trying to show compassion by making love to a cruel, toying parasite, a murderous beast, an invisible being of self-loathing, or a sorcerer who will curse as soon as look at you. Very appropriate to the Xander type (anybody recall the season finale where he went head to head against a godly Darkest Self Willow with no power at hand but nostalgia?)
I third Renee's Darkest Self! Also I like "You tend to find yourself at the center of things. Whenever a character's backstory requires them to choose another character, and you're chosen, they mark experience. (Limit one experience per character, even if they can somehow select you multiple times)" a lot better than the way it was originally written, because having other people choose to have their options apply to you really drives home the way in which you matter to them more specifically.
I think that this playbook would be greatly helped by distancing itself from that other Mortal Playbook. Change the name and get rid of the moves that overlap, or fiddle with them until they are unrecognizable. Could be cool to present a supernatural option of some kind, much like how the Queen can be a bitchy cheerleader or a parasitic, cult, hive mind. Not sure exactly what it could be, though. Hmm.
I've got a few ideas for moves if you are interested. Off the top of my head...
-Who invited you?: When you accuse someone of not belonging to the group (whatever the group might be at the moment) in order to get them to leave, Spend a String on them. If they stay anyway, gain 2 Strings on them.
-Not so Tough: When you face someone head on in an area in which they are clearly more skilled or otherwise have a huge advantage, Roll+Volatile. On a 7-9, you are beaten badly, but can hold your own long enough for someone else to act. On a 10+, you can hold them off and delay the inevitable as long as you need to. In either case, if someone else takes advantage of your opponent's distraction, they take +1 Forward.
-Latest Humiliation: When another character goes out of their way to insult or belittle you publicly, they mark XP and you gain a String on them. When you are competing against someone, and it's a huge rout (with you as the loser), gain a string on the winner.
Are you still working on this? I'd be interested in making it all beautifully official playbook formatted if you don't mind.