I feel like the core trio in that novel is Alex, Sam, and the specter of Alex's old mentor, Professor Vance.
Alex carries the whole weight of the guilt from the accident that ruined Sam's career, and his journey from self-destruction to trying to make amends is so raw. Sam isn't just a victim; her bitterness is so palpable it's almost a character itself, and watching her thaw is painful in the best way.
Vance, even though he's absent for most of the present-day plot, is the ghost at the feast. His philosophical letters to Alex frame the whole idea of whether redemption is even possible, or just a selfish hope. Honestly, the side characters like Alex's sister feel a bit underdeveloped, but I guess they serve their purpose.
The real dynamic is between Alex's desperate need to fix things and Sam's reluctant, gradual acceptance that some wounds never fully close, but can maybe stop bleeding.