Rook, who is what he is.

Rook is an Eternal Seeker - he's forsaken a concrete destiny for an undefined ending to his story, and he hasn't known what he really wants to achieve until maybe a level ago. He's also forsaken the pursuit of grand power, in exchange for complete freedom to invoke whatever power he's found necessary to do his job as a Stormcrow. He's wielded martial, divine, primal, arcane, and even psionic force in his duty, and his final powers - one sorceror utility (Platinum Scales) and one epic destiny feature (Ceaseless Guardian's Irrepressible Defense) - will cement his goal of being the indestructible core of the Stormcrows.

That said, once the Stormcrows disband, that's the end of Rook's career as far as he can see. He's accomplished the beginnings of the war college in New Arkhosia, and ensured that the next generation of warriors will be there for the Shard when they're needed. Rook's ultimate destiny may be to either retire to a secluded location on the Shard, or get Cas to permanently alter his body and just take on a new identity. Or, depending on circumstances, he may fight his last battle as the Stormcrows do what needs to be done. He knows that despite his sins, Bahamut apparently holds him in high regard, and dying to protect the Shard might be Rook's way of repaying his god for his blessings.

Rook's got a lot of flaws. One big flaw is that he's too quick to compromise his own morals. He'll get brutal and ugly if a friend is threatened, and he'll easily capitulate to other party members if they decide to use cruel and unusual tactics to accomplish something. This is the primary reason he feels guilty towards Bahamut - he's supposed to be an exemplar, and strive for the whole 'justice and protection' thing, but that just doesn't happen with him. He's also failed a lot of people important to him - his parents, his sister, Prudence, Xan on more than one occasion. He's a bit insecure - he started out his adventuring career running away from responsibility and looking for fun and glory, and now he's trying to catch up on all that responsibility he didn't take in the first place.

His strengths... Despite all that taken into account, though, Rook is aware of it all, and he's trying to do better. At this point in his life, he feels that he can go to his grave... not with a clear conscience, but with the knowledge that he did his best with what he had, and his best was pretty goddamn good, all things considered. He also has developed the ability to use just about whatever resources he can find in order to do what he does better, exemplified by his breadth of powers across the range of classes. Most importantly, he knows what's important to him, and he sticks to it no matter what.

If Rook did make it to a pantheon (which he'd rather not - he wouldn't want to take worshippers away from Bahamut), he'd probably become a deity of self-improvement. Acknowledge your mistakes, and learn from them; perfection is impossible, but still a worthwhile goal to aim for; try everything, and at least one thing will work.

tl;dr: Rook's an indestructible force with mommy issues and no well-defined destiny. His ultimate end will depend on the events of the Stormcrows' final farewells.