A Soul Reaper, aka, a Shinigami or a Death God, is from the anime/manga Bleach. They are spirits that help ghosts pass onto the afterlife, and that hunt down evil spirits that have been corrupted into "hollows" and started attacking innocent ghosts and people. They come from another dimension called the Soul Society (otherwise known as the afterlife) and regularly patrol the human world to protect ordinary people from hollows. Soul Reapers (and hollows) are invisible to most people, so very few people realize they exist.
At the start of Bleach, the main character, Ichigo, one of the few people that can see hollows and reapers, is attacked by a hollow. A soul reaper named Rukia tries to save him but is critically injured by the hollow, and in desperation she transfers her powers into Ichigo. His soul pops out of his body, becoming a Soul Reaper himself, and he wastes the hollow. Turns out Rukia gave him more power than she meant too, and she won't be able to fight or return to the soul society for a while. Also, Ichigo's transformation into a soul reaper has somehow become permanent, which she didn't intend. So she conscripts him into helping her fight hollows as a "substitute soul reaper" until she recovers. Basically, he goes about his life as normal, but when Rukia senses a hollow, he pops out of his body, hunts it down, and then climbs back into his body afterwards.
In Billy vs. Snakeman, you can become a Substitute Soul Reaper yourself after getting the required items and passing the special mission, helping out a temporarily powerless soul reaper named Shorty (the BvS equivelent of Rukia). You can go on special "Reaper-only" missions, where you can only take Reaper allies and use special Reaper jutsus. I don't know anything more about it than that though, as I haven't actually become a reaper yet.







