It's the black-skin-yellow-eyes thing. Seriously, he's nothing like him.
Tendril's a shadowmorpher; he has an alternate form that's a semi-fluid and amorphous creature of pure shadow. He can extend whiplike tentacles out of his mass, squish himself into near two-dimensionality, and...well, otherwise behave a lot like a Shadow Heartless. He can't teleport, he has more than three digits per hand, he has no tail, and his hair is a semisentient mass of writhing little tendrils. ^_^;
Not much like Nightcrawler at all. (He's also not Catholic.)