Namor

Another Marvel hero who definitely has the necessary willpower to successfully wield a Green Lantern power ring is Namor the Sub-Mariner.

A hybrid of human and Atlantean DNA, Namor is also a mutant and, as such, has a very unique physiology which includes great stamina – but, even when that runs out, he has proven that he is prepared to keep fighting using his willpower alone.

Such is the level of his willpower, when Doctor Doom was attempting to telepathically control the world’s entire population by using an amplified Purple Man as his pawn (Doom being someone who knows a hell of a lot about having great willpower himself), Doom took precautions because of the presumed resistance of just one character – and that man was Namor. Namor has also mentally resisted the powers of the extremely potent Serpent Crown – to the extent that he was actually able to escape its grasps and remove it.

Namor is already very powerful, so with the powers of a Green Lantern he’d be even more worthy of ruling the seas than he is now.