| First of all...
OH MY LORD, that Mytoss drawing is hilarious! It's too perfect for words. "The future is safe" on his helmet = so much win it's not even funny. The singing tree logo on his shirt is great, too.
Dark Millie looks pretty rockin', though I'm not sure how I feel about the horns, to be honest. I think she'd look cool enough without them. I've always seen her as less of a demon-from-hell and more of a possessed or messed-up version of Millie. The look you have there is badass, though, so I've got no complaints. I think the skin color, hair color, eyes, etc. look pretty cool. But it's up to you.
However you want to implement Dark Millie, storywise, is up to you. Whether she's a separate character, or whether Millie turns into her (i.e. Fei/Id in Xenogears) is your call.
As someone mentioned earlier, maybe there's a way to work in some puppies that are Millie's friends and allies? Shiba puppies are the cutest. Maybe there is a playable puppy who is one of the good guys? | 