Who's this? The appraiser of the Tree(house) of Harmony. He makes sure it's healthy and whatnot. Oh, what's his name, you mean? …Haven't decided yet. But! I've got his basic character outline. For a hundred years, he's been the appraiser of the Tree of Harmony, having been entrusted by Celestia with some of her power in order to protect the tree if need be. And when the tree recognizes it is in danger — or horse-God forbid, withering due to disharmony among the Element-bearers — it will recognize him as its protector and lend its power to him to destroy whatever is threatening it. In other words… he does things that aren't exactly harmonious… but it's fine since it's in the name of harmony.
Yeah, who would've guessed that the MLP universe has its own version of the angel Mastema?