Rainbow coin/medal is a compensation one of the system in DMM FKG hahahaha.
You get it from dupe 5* / 6*.
You also get extra medal for choosing dupe girl with special ticket I believe.
Visit http://flowerknight.wikia.com/wiki/Gacha for more intel.
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I was wondering who is Hollyhock that many want her badly.
So she's a rainbow for Nutaku current event.
What good is she except her cool and smart appearance?
Her active similar to other rainbow.
Her passive is crystal drop, very less benefit to use, and no bloom yet.

It's just like Hop time that many go berserk for her.
Somebody on discord do lost 300 or 400 gems to get her.
I'm very... confuse...