So, as of today's strip (4/24), Amber having something like Dissociative Identity Disorder is as confirmed as it likely is going to get, given Willis's general dislike for labelling his characters' mental states. This is a big revelation for poor Danny who, I suspect, had previously thought that dating Amazi-Girl, not Amber, was either a kind of kinky costume thing or even a secret identity thing.
Here's the weird part: Both of the alters living in that body like him a lot. This could lead to one weird romantic triangle. It will be a test of his feelings for both Amber and Amazi-Girl whether he's willing to take on that complex relationship.