I actually have 2 rainbows, but neither is in my Helper squad. Cactace is lvl 70 now, but lvl 60 Edelweiss in my main squad is generally more useful, so I see no reason to switch the two around. I could level up Edelweiss to 70 I guess, but I'm working on evolving Peach for a speshul waifu squad. And lvl 60 Edelweiss is plenty for all the maps.
The other rainbow is Baccara, but I just got her, so it'll take a while to level her up and replace St Paulia.

Actually, my helper squad could use a big overhaul anyway, but I think I'll sort that out once I maxed Baccara and some other units. Right now, it's just designed around stopping just before the switchnode in 12-3 and having two units that can double-hit Wealthy Flies. Maybe when my Waifu-squad project is completed, I'll make that my Helper squad. Both for fun, and as dedicated single-target squad to take down bosses.