Sidwell

This is pure speculation, I have no actual spoilers to back this but this is my theory.

Sidwell is a Cassadine.

The Cassadines haven't been heard from in some time and all of a sudden the Cassadine lawyers in Greece take back Ava's divorce settlement as Sidwell comes to PC.

Sidwell bought Spoon Island.

Sidwell really enjoyed toying with Lucky and seems to already be showing hints of an obsession with the Spencers in general.

Sidwell had diamonds which is reminiscent of the Ice Princess.

I also have to wonder if Lucky knows something that he hasn't revealed because Jason was going to kill Sidwell back in Africa but Lucky stopped him and they never addressed why. Lucky doesn't condone unnecessary murder but he himself has killed people who are dangerous and pose a threat and Sidwell is certainly dangerous and would have killed them so why did he stop Jason?