Completely ignoring the mechanical problems (removable bones and the like) if the character doesn’t make sense in the setting they shouldn’t be playing it there. As a caveat to that though, if they’re dead set on playing that Super smith bros tee shirt, perhaps there’s an alien race they can play that uses those mechanics but fits the setting. Not saying you should allow it, but something to keep in mind.
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Did the other players get equally problematic abilities? And if not, why not just play a Super smith bros tee shirt, where he’s actually alive with a beating heart and a brain, just being kept alive by some kind of mechanism. Maybe he has to drink some formula periodically that keeps everything going, or get an oil change/tune up. Basically, he’s always dying and can never take a vacation from upkeep. He could be mostly exposed bones with some patches of thin skin and flesh where it matters.