I guess I could turn a blind eye.
|—Guicho to Mary Beth|
Guicho  is a museum office guard.
Guicho wears a normal security guard uniform. He has a white beard and mustache (besides sideburns), brown eyes and a big crooked nose.
Guicho has fun scaring the Detention Kids when they visit the museum. Although at first he did it to prevent them from entering an area prohibited by "the rules of the administration"; demonstrating that he cares about compliance with the rules and (maybe) for the safety of the kids.
Guicho is seen for the first time when he suddenly appears, scaring the Detention Kids and earning a look of disapproval from Mary Beth. The guard warns them that "the rules of the administration" prevent him from letting them through. However, he is easily convinced by Mary Beth to make an exception with her and the kids.
He is not seen again until Mary Beth finishes telling the story of Manolo, María and Joaquín to the kids. He is moved and sheds a tear with the end: "That love, true love, the really, really good kind of love never dies".
Once the kids leave on the bus, Guicho reveals himself as Xibalba in a human disguise.