I'll keep you informed about the coins
I was talking to a friend of mine about that today (the injuring him thing) and it seems like she's the only one who can. My guess is, she's a nephilim. She mentions her mom is still around, but not her dad. The war in heaven would have been pointless if angels couldn't hurt each other, so if we take that idea and run with it, she's part angel.
Obviously she's not aware of it, otherwise the whole "what are you" part of the show would be kind of redundant, but it'd be an interesting idea.
Other reasons we came up with were his feelings for her are turning him more human and he's slowly going to lose his angelic badassness over the season. Love conquers all seems to be a bit on the romantic side but you can tell they are working the female audience on the show, so it makes sense in a romance novel kind of way. Hell, she's probably the first person he ever asked "Please" to in his existence, and actually meant it.
Last option was God, but a literal Deus ex machina twist like that would be a little bit crazy, even for this show.
We only see her slapping him in the trailers, so besides being the one who shot him, there's not much there to say someone else ~can't~ hurt him, but the evidence so far only points to her being able to do it. It'll make for an interesting story at the least.
Chris