9. Blendin Blandin
Blendin Blandin is a minor villain in Gravity Falls, who is able to time travel and camouflage with any setting he's in. The Pines twins first encounter him in the episode The Time Traveler's Pig when hes allows to use his time travel device, resulting in him losing his job as an operative for Time Anomaly Removal Crew. Since then, he was in a couple more episodes across the second series.
However, he's actually been present much more than you realise as he casually appears in the background of several episodes, including the first episode Tourist Trapped. In fact, Blendin Blandin could technically be present for every single episode as he is able to camouflage himself, therefore going unseen amidst the action of Gravity Falls.
He's not the only 'hidden' character as Agent Powers is sometimes seen in the background of certain episodes, as it is learned later in the season that he is there to keep a close eye on Stan.