Nadine Zenobia Grex is my nominees.
Nadine has a good art, keep levelling and auction it to earn some black Time Elixirs or Green Potions.
Zenobia has a hidden Iron Wall skill which is good for battle arenas.
Grex is maybe not useful but because he has a speed of A, its for a win.
Look for Angela, she is better than Grex. FYI.