Character Arc
When we first meet John Reese he is a broken, bearded drunk riding the subway and waiting to die. A decorated Special Forces soldier turned CIA killer, he lost the one person who anchored him and walked away from a life of black operations with nothing left to lose. Finch finds him in this gutter and offers him a reason to get up: a chance to save people instead of erasing them.
Given the Machine numbers, Reese becomes the team's blunt instrument, the man in the suit who appears out of nowhere to shoot kneecaps and rescue strangers. Yet the work reawakens something he thought was dead, and beneath the icy precision a wry warmth and a fierce protectiveness emerge. His bond with the wisecracking detective Joss Carter, and later his guardianship of the team, give his violence a moral spine.
By the final season Reese has chosen this purpose completely, accepting that a life spent guarding others may demand his own. His arc closes on a rooftop in return 0, where he stays behind to buy his friends a future, trading places with Finch in one last act of loyalty. It is the redemption he never believed he deserved.