The Keeper of the Bridge of Comprehension is the quaternary antagonist in the Phineas and Ferb episode, Exacaliferb. She is the fantasy counterpart of Esmeralda Poofenplotz.
The Keeper does not allow anyone to cross her bridge unless they answer her riddles, seemingly oblivious that the water under her bridge is shallow. She gives some complex instructions about her riddles such as categorizing them like a game show and that answers should be in the form of a haiku. She is only seen when a group of questers (Phineas, Ferbalot, Isabel, Bufavulous, and Baljeetolas) attempt to cross her bridge to find the legendary sword Excaliferb in order to fight against the evil wizard Malifishmertz and his army of meat monsters. The Keeper then gives the kids an riddle to answer, but Baljeetola points out that the creek below the bridge isn't really shallow, so the kids decide to cross there instead, much to the Keeper's anger. However, when Baljeetolas gives in the correct answer to the Keeper's riddle, she decides to let them go.
She is seen again when Candavere (the fantasy counterpart of Candace Flynn) arrives in a mutant form, much to the Keeper's horror. Rather than giving Candavere a riddle, the Keeper fearfully allows her to pass through.