What is the name of the former stormtrooper who joins the Resistance?

The story behind the answer

Designated FN-2187, a First Order stormtrooper, he defects after the Jakku village massacre, helps Poe Dameron escape the Star Destroyer Finalizer, and later commits to the Resistance. Taken from his family as a child and indoctrinated, his crisis of conscience marks one of the sequel trilogy’s key character arcs, portrayed by John Boyega. The code “2187” nods to the detention cell in A New Hope.

By contrast, Poe is already a Resistance ace pilot; Rey begins as a scavenger on Jakku and discovers her connection to the Force; and Kylo Ren is a dark side leader of the First Order.