Yeah so I'm pretty sure you can just use whatever the hell name you want. I made a second account and used one of my 3 guest passes on it (since apparently guest passes can only play with guest passes and I wanted to play with my guest numbers) and was able to use the same name as a character from my main account. I guess it's possible that the names of guest accounts and normal accounts don't overlap, though.
Also I joined a public game with my main account and my name showed up as Lopen rather than my character's name? That's kinda lame but whatever I'm probably going to mostly be playing this as single player anyway.
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No problem!
This is a cute and pop genocide of love!