Jeb is a dummy that Adam made when he was in 9th grade (2000) during winter break. The first model of Jeb was made from a pair of snow pants and a long sleeve shirt with various clothes stuffed in him. Jeb gradually evolved, and was given a head a few months later. The clothing inside of him was replaced with cotton and clothespins currently hold him together.
If you look closely, you can see Jeb in most of the AML videos.