Gina and Yuyu are pretty solid, though Gina/monks in general are a niche unit.

Monica is also decent, but you'll generally use her more for her passive of increasing demon crystal drop rate.

Elizabeth is one of the best plats and is essentially on par with Nanaly in terms of burst damage. Level and awaken her immediately.

Conrad... well, if you don't have any other lightning rods, he's fine.

Robert is only for male-only maps, really. You'll generally want to stick with Alissa/Iris and other female healers.

Themis is the worst princess by a fairly long shot, even if mincosted. If you don't have Claudia or another princess at all, you might get use out of her, but she's not a good lightning rod and even Conrad would probably be better suited to that role.