Well, the only person I can think of that could be somewhat similar to Hel would be Ghetsis (with his completely black right arm). Their genders are different, but that's a trivial difference. Hel rules the underworld known by the same name, and of course, Ghetsis is best represented by the The Devil arcana (and he had N's Castle built underground!). I think Ghetsis did want to try re-creating the original dragon with Kyurem + Reshiram/Zekrom...but we all know how that turned out. Maybe the main antagonist of X and Y will try to succeed where Ghetsis failed.
I think Nidhoggr and the Midgard Serpent are two separate beings, since their articles don't mention each other.
I think Nidhoggr and the Midgard Serpent are two separate beings, since their articles don't mention each other.