Hi all.
I started my playthrough on day one when Steam was being wonky with the achievements, so not getting this at that time didn't bother me overly much. However, as time goes on, I keep seeing this as an unearned achievement and it is starting to bother me. I doubt that this is the case, but is there some quest or action that I need to perform to unlock this? I used the key that I obtained from the backer portal, and have gotten the cloak, ring, and pet with no issue.
I know that this issue could possibly be laid at the feet of Valve and their achievement system, but before I submit a ticket to be ignored, I figured that I would come here first to see if anyone else has experienced this and might have any possible fix short of contact Valve.
Thanks in advance.
Edit: Fixed by making a new character as suggested below.