What role does Armin play in the final arc of Attack on Titan, and what decisions does he make?

In the final arc, Armin becomes a key negotiator and strategist in the alliance against Eren. He works with the remaining members of the Survey Corps and their allies to devise a plan to stop Eren's Rumbling. He makes the difficult decision to allow the alliance to kill Eren, believing it's the only way to save the world, despite the immense emotional cost. He then dedicates himself to building a future based on peace and understanding.