Mammon is a Thunder SSR Hime who will be available in the third Miracle ticket. Her estimated release is around mid-June. She's extremely powerful and basically defines Thunder as an element, at least until mid-November when Brahma gets her Awakening. Well, even after that, Mammon continues to be really damn good.

Perun can be really good against very powerful boss Overdrives - use her damage cut ability with Raiko and you'll reduce incoming damage by 55% for one turn. In content which doesn't hurt you? Anything will do.
Same goes for Aphrodite by the way. Since you have Kirin, running Aphrodite hurts your damage output by a lot. If the content hurts you and you need the heals, sure, use her. Otherwise, replace her with a Thunder R.