The Guardian of Secrets is the main antagonist in the episode "Diary". She appears when Rigby opens his eyes during a ritual. She blows up only when everyone who was involved in the unfinished ritual reveals secrets with deep meaning (Skips revealing why he skips, Mordecai revealing that he lied to Rigby about his mattress never coming and that he stacked both his and Rigby's on top of each other, and Rigby revealing that he thinks Eileen is hot without her glasses on).
She was not revived by Garrett Bobby Ferguson Jr. in Exit 9B.
She may only look like Margaret because she is the guardian of Margaret's diary; in other words, if someone owns a diary, their Guardian of Secrets may look like the owner but 5-10 times more powerful.