When Joe returns to earth with the earth pass that 22 begrudgingly gave him, she enters a depression, believing she has no purpose, and goes to the Astral Plane where she eventually becomes a lost soul herself. Moonwind then mentions how he for a time was a lost soul, apparently once having an obsession with the game Tetris. Moonwind gives some insight on the lost souls and then a soul, determined to find something with a metal detector, is witnessed becoming a lost soul. Joe and 22 board Windmoon's ship along with him and his crew, and on the way to a thin spot in the Astral Plane, Joe takes notice of even more lost souls, noting how there are so many of them. Once the soul was free, Moonwind helped the soul return to his body where he happily destroyed his computer and tried to get his co-workers to do the same. Moonwind's crew make some music so as to free the lost soul from its trance and its body dissolves to reveal the soul of a hedge fund manager. However, they were saved by Moonwind and his crew who restrain the lost soul with a net. Joe ends up alerting the lost soul to his and 22's presence and it chases after them. When Joe and 22 arrived in the Astral Plane, Joe soon witnessed several lost souls, one in particular moving about and continuously saying 'make a trade'.

“But if you do it to the exclusion of everything else in life, you might become a lost soul.” Role in the film “Some people get caught up in something that isn't necessarily bad-cooking, video games, art,” says director Pete Docter. Lost Souls wander in The Astral Plane while their Earthly selves struggle to break free of obsession.

Lost soul aside wikipedia