Edelweiss and Lavender seem like okay picks. Their active skills have a low activation rate, and their abilities aren't great but they look like they hit hard.
Monotropastrum I may have to get just on the basis of her flower being awesome. A hyperparasitic flower called "Ghost Pipe" and she talks to ghosts? The only thing that would make that more perfect is if she actually was a ghost. Maybe her friend could be Hypopitys monotropa, a Tsukumogami inhabiting a weapon or something. Such a cool flower.
IMO, Edelweiss is the best event girl right now due to the fact she can be bloomed. Her bloomed form, stats-wise (which is important for whaleship) beats some of the weaker rainbow girls.