Jason-freaking-Blossom. Yes, if you need clarification Cheryl's DEAD brother.
See the gang's shock reaction to the reveal in the epic season finale trailer below:
It remains unclear how or if Jason really is behind it all after this week's penultimate episode "Chapter Fifty-Six: The Dark Secret of Harvest House."
If it truly is Jason we wonder if the show will take on a supernatural theme for season four and finally gives us a Chilling Adventures of Sabrina cross-over. Fingers crossed!