The Corn costume guy is the owner of a roadside attraction corn maze location on Oregon.
Before the twins' visit
The Corn Costume Guy is one of Stan's tourist trap competitors who prank the Mystery Shack every year. Last year, they duct taped Soos to the ceiling, leaving him there for 78 hours.
In "Roadside Attraction." He was seen carrying a box of corn at the maze while Dipper is taking with a girl, meanwhile Grunkle Stan releases a bag of corn weevils in the maze before they leave.
When Stan, Dipper, Mabel, Candy and Grenda return to Gravity Falls, the entire shack is messed up by Granny Sweetkin and the Upside Down Town manger, the guy breaks one headlight on Stan's RV before running off with the others.