SPOILER: In the second credits scene, 1,000 years ago, Gang-lim's father is seen dying, and is approached by Yeomra about taking over his position. He does so, taking on the appearance of the former Yeomra. This means, all along, over 1,000 years as a Reaper in the afterlife, the Yeomra Gang-lim has been serving all along has been his own father he was desperately seeking the forgiveness of.
SPOILER: In the first credits scene, Kim prepares to be reincarnated, but is approached by Yeomra about "working together," implying he may stay in the afterlife and take on Gang-lim's former position.