Over the course of its lifespan, the Michigan Soul Eater stole the souls of these people and trapped them in its Nest. In the real world, their bodies withered and died while their souls withered as well, becoming deformed husks of themselves. Every time the Soul Eater moved its nest, it took the Sad People with it to help keep it fed.
In 2008 or 2009, hunters Bobby Singer and Rufus Turner investigated the case of Will Henderson and later his mother Mary who were attacked an entity that left handprints on their legs. Bobby and Rufus eventually determined that they were dealing with a Soul Eater and Bobby was dragged into the Nest while he and Rufus were trying to trap the Soul Eater. In the Nest, Bobby encountered Will who told him about the Sad People. However, Bobby was not yet able to see them even though the Sad People surrounded him. As Bobby and Will made their way through the Nest, they became separated and Bobby was distracted by the appearance of the Sad People around him. Before Bobby could recover from his shock, he was attacked by the Soul Eater and possessed.
In 2016, Dean Winchester allowed himself to be taken into the Nest in order to kill the Soul Eater. While in the Nest, Dean encountered Kat who informed Dean of the souls surrounding him. Shortly afterwards, Dean was possessed by the Soul Eater, watched by the Sad People.
After the death of the Soul Eater, the Sad People began to disappear from the Nest one by one and move on to their afterlives. A pleased Kat informed Dean that the Sad People were leaving. Shortly before returning to their bodies, Dean and Bobby's souls briefly encountered each other while they watched the Sad People disappear, but Dean and Bobby weren't sure if it was real or not afterwards.