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Given the descriptions of the half-demon classes and the Silly tone, I think the personalities I'd most like to see on our protagonist are either being the tease of the party, or forgetful and distractable. Either of those demeanors, I think, would work well in this context.
Physically though, early 20s works well for me too. A cosmetic battle form of accessory demon characteristics (wings, tail, horns) triggered by intense emotions sounds nice too.
I also have a few relevant pictures for us to mull over.
