The only thing that had kept Evan sane in the after life was the knowledge that Barty would follow him there eventually. (Knowing Barty, sooner rather than later.)
So, when years went by and Barty didn´t turn up, Evan became restless. As a young soul he didn´t have the power to visit the world of the living immidiately and when he finally managed to check on Barty it was too late. There was no Barty to be found. Not fully at least, only the shell of the boy (now man) he loved.
Evan raged. But there was nothing to be done. Barty´s soul was gone, swallowed by that fucking Dementor.
Now the question was: how to get it back? - Kill the Dementor of course!
But how does a ghost kill a demon? - seems like he has some haunting to do.
The plan for now:
-find someone to haunt
- get them to kill the Dementor that kissed Barty
- release Barty´s soul
- kill Barty and take his soul to the afterlife
- be dead happy ever after