Rainbow Twins appears in B3F and B4F of the Secret Laboratory within the Midnight Channel on rainy days. They will attempt to use Stagnant Air to increase the chances if status ailments of their Physical attack skills sticking. After being defeated, they may drop Cascade String. Selling the Cascade String will unlock Uin Haori female armor in the shop, which has 10 points less Defense than the best available female armor from Secret Laboratory but adds a Resist Confusion effect.
In Golden, it also appears in B7F and B8F. It may drop Meteorite, Rose Quartz or Buffalo Stone.