The Guardian of Secrets is the main antagonist of the Regular Show episode "Diary".
She appears when Rigby opens his eyes during a ritual. She vanishes only when everyone who was involved in the unfinished ritual reveals secrets with deep meaning. She tried to kill Mordecai, Rigby and Skips for looking at Margaret's diary.
She looks like Margaret, but she is much larger, has a red glow, and wears a warrior-like outfit.