Bio: Rainbow was a part of the terrorist organization known as I.C.O.N. Their main goal was to create an army of undead monster super soldiers. To accomplish, they planned to kidnap Frankenstein's Monster and study his brain, as he managed to retain his intelligence when he was created, as opposed to the creations of I.C.O.N. who had very limited intelligence after being brought back from the dead.
Rainbow was only ever seen giving orders to I.C.O.N. agents Indigo and Cardinal and only ever in shadow. His true identity has never been revealed.
Notes: It's unclear if the cancellation of Frankenstein with issue #18 was the reason Rainbow was never seen again or if the writers were simply done with the I.C.O.N. story line and didn't plan to pursue it any further.