Okay, so we've seen Phoenix's different forms, but what's it like for her when she transforms? Is it painful? If so, then (as the great Baymax from Big Hero 6 put it best): "On a scale of 1 to 10, how would you rate your pain?"
It depends on which form she's taking. Her transformation from her humanoid form into her monstrous, self-immolation form is surprisingly painless! Honestly, it feels powerful and borderline addictive. She does burn to ash after using it, though, so that's probably a 100 on the pain scale. Her rebirth form isn't painful at all, since she was a pile of ash just before transforming lol.
Here's some art I got of her different forms as well! Feels appropriate to share them with this question XD
Base reference sheet: @fishymom-art
Finale of Fire and Ash: @lendubsofficial
Rebirth: @mimikip-stuff
