[AUTO]:[Choose one of your other rear-guards, and put it into your soul] When this unit is placed on (RC), you may pay the cost. If you do, search your deck for up to one card with "Zeal" in its card name with the same grade as your vanguard, ride it as [Stand], and shuffle your deck. If you rode, choose a card from your soul with the same grade as your vanguard, call it to (RC), choose one of your opponent's vanguards, and it gets [Power]-3000 until end of turn.
From Heroes to Villains, this time the focus is support that improve a deck dramatically, as opposed to it's focus or boss. In Japan this unit was originally called Evolution Monster, Darwin, which had that name also been on the English card that would be just perfect. Taste Darwin's evolution theory *****es! But alas, that was not the case. T_T
Missed name opportunities aside, Evolution Monster, Darvain is a support unit meant specifically for the Galactic Beast, Zeal deck, now "upgraded" to New Era Beast, Zeal with the release of Cosmic Roar. Evolution Monster, Darvain's skill improves consistency of the deck and of achieving the ride chain successfully while also draining an opponent's power enough to trigger Fusion Monster, Bugreed creating a nice combo piece at any phase of the game, but best used early. How it achieves this is by taking one of your other rearguards and putting it in the soul to Ride a Zeal of the same name as your current grade and pull out your previous Vanguard that would have then moved to soul. This means Evolution Monster, Darvain is virtually cost-less, the only real cost being that you had to call him, but is that really a loss once you think about it?
Zeal is considered the weakest of the decks that were supported by G-EB-01 Cosmic Roar but I would debate that, and with support like this, Zeal is a sleeping giant that should not be underestimated, not when there's also Dark Superhuman, Omega that gives Zeal's support a turn in which is can push for more than just raw advantage gain.