Jeff is a gnome in the tv series Gravity Falls. He is the leader of the Gnomes.
Jeff and four other gnomes disguised themselves and pretended to be a guy named Norman in order to date Mabel. When they were alone, they revealed to Mabel who they really were and told her that they did this because they are looking for someone to marry to make her their queen. But when Mabel suggested that it's not going to work out, Jeff kidnaps her and try forcing her to marry him and the gnomes, but was saved by Dipper. Jeff is able to make all the gnomes form a giant monster. When tricked by Mabel he said he'll get them back for this, hinting the gnomes will return.
He later reappeared in the episode Gideon Rises where Dipper and Mabel tried to recruit him and his army to fight 'Lil Gideon (and also accidentally discover that he bathes in squirrels in his free-time). Jeff refuses, but the twins claim that Gideon is a girl and offers to let Jeff marry him in return for his help, to which he agrees. The gnomes quickly occupy Gideon's strong-hold, but are defeated and enslaved after Gideon discovers that they can't stand whistle's. The gnomes turn on Dipper and Mabel, and the third journal falls out of Dipper's vest during the following struggle. Gideon claims it for himself and orders the gnomes to remove the twins from the property. Jeff then retreats into the forest with the rest of his troops, furiously telling the Pines-siblings to never try to employ him again.
Jeff returns with his most devilish plot yet in the 3DS game Gravity Falls Legend of the Gnome Gemulets, when he turns to Dipper and Mabel for help to recover his so-called valuable Gnome Gemulets that the forest needs for surviving; and, since his gnome comrades are not willing to help because they lost all respect they had for him when he failed to get Mabel for their queen and the twins fooled him with the whole Gideon scheme, he has no other people to recur to. Dipper and Mabel endure great hardships to recover the little, yet valuable gems; an ordeal that Stan approves since he also promised them great quantities of Gnome Gold if they succeed. However, when all of them are recovered, Jeff reveals it was all a scheme to recover his lost leadership, using the Gemulet's secret powers to become Dark Jeff; and that the jewels did not belong to the gnomes in the first place, as they all housed a powerful creature ancient creature called Gramularth. Jeff takes control of the Mystery Shack and uses the town center to build his new domain.