Ok, so to sum up: he's a monster, he's been around long enough to qualify as an adult even if that was a thing for monsters (you'd estimated 100 years ish?), and he just Looks That Way in the interest of being unsettling because that's his purpose. Makes sense to me!
For Nightshade, I don't know anything about the series but my co-mod Dara is more familiar and we've decided based on what we know that your logic on his age checks out. He would be ok to join but would take on a more period-appropriate aesthetic and size if necessary! Just for the sake of not scaring the locals lol.
For Robert, I would say that he is probably also fine but to use your best judgement. If his personality reads like that of an adult canonically and you think it would be easy to make this come across in your writing, then you're good.
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For Nightshade, I don't know anything about the series but my co-mod Dara is more familiar and we've decided based on what we know that your logic on his age checks out. He would be ok to join but would take on a more period-appropriate aesthetic and size if necessary! Just for the sake of not scaring the locals lol.
For Robert, I would say that he is probably also fine but to use your best judgement. If his personality reads like that of an adult canonically and you think it would be easy to make this come across in your writing, then you're good.