I think I'll go ahead and give my vote to Evangeline. The button is certainly powerful, but a frozen Coop isn't going to be pressing it. Eva only needs one spell to make that happen, and honestly, I don't really see her not getting it off. And while it's generally an argument I avoid, Eva has the body of a 10 year old girl. Even if Coop notices her right off from up in Megas' cockpit, that's not exactly something that inspires intimidation and urgency.
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Dying breath, light my blade and sing in shadow! Omega!