If a PK file is corrupted/crashing is being encountered the following should resolve it:
1. Go to C:/users/"YOURPCNAME"/saved games/portal knights
2. Find your save files -- try to find characters.backup and change the name to characters.pksave (make backups first, just in case!)
3, This should assist in recovering your lost character data.
Additional information can be found in this thread on the Steam discussion forums: