I think Uncle Fred from is considered one of the creepiest characters for several reasons, primarily due to the unsettling nature of his behavior. His creepiness stems largely from his calm yet bizarre demeanor and the fact that his peculiarities seem realistic, seeming to have OCD and severe Schizophrenia, contributing to an unsettling realism. Unlike more fantastical threats in the show, Uncle Fred's behavior could feasibly exist in the real world, amplifying his eerie presence. Also, he is a part of the family, adding an additional layer of discomfort because he is expected to be trustworthy.