Known by his nickname "The Disposal", Seymour is an extremely voracious young boy from P.S. 119 who has an unbelievable appetite and can eat grand portions of food with ease. Because of his legendary appetite, it is rumored that Seymour has two stomachs. He was the main villain of the episode Eating Contest.
His enormous appetite proved to be his ultimate downfall---while Arnold had spent days practicing on how to eat a lot quickly but safely, and then not eat or drink anything except water until the contest, Seymour stuffed himself before the contest even started and shoveled down food as fast as he could look at it. This combination filled him up too soon and Seymour ended up passing out from a major stomachache before he could win, allowing Arnold to win the eating contest.
Seymour is another character in the show where his last name isn't known and he never talks at all.