Kleimann and Orion are both decent dark units. They aren't as good as the top adventurers of the other elements, but they're the best units in the shadow element right now. If you need a dark unit immediately, I'd suggest going for one of them. I would have suggested Nefaria, but bows are a really unbalanced weapon type as of now. If you aren't in a hurry, I'd suggest saving up wyrmite and waiting for the dark banner which I assume will be the next showcase. You're sure to get a good dark 5* there.