If anybody was wondering why I didn't repost the sketch that TwilightSaint did, this is why: Mystic, fully decked out in glorious plumage. The way I see it, Mystic is naturally born as a feral orca who eventually fine tunes his shapeshifting abilities to take on an anthro form. Then as he matures and strengthens his abilities, he begins to add a pair of wings and thus takes on the archangel form. So I think the wings are here to stay, though they tend to stay unrendered in most situations unless they're called for.

Art by  TwilightSaint

I'm gonna change you like a remix,
Then I'll raise you like a phoenix,

-Fall Out Boy, The Phoenix