Elmo is a middle-aged repair man who makes a living repairing old appliances or tearing them apart to sell for profit. Therefore, he is feared by all who are sent to his shop. Radio tells the other appliances that Elmo seems like a nice-enough fellow, which describes that he is their new master, but then, what Elmo did to Blender made them found out that he is cruel towards them, which describes that Toaster and his gang do not need a new master after all (although one must note he does not know that they are alive).
Elmo shows up in the swamp where Toaster and his gang nearly drown and rescues them from a cruel fate. He then loads them in his monster truck and takes them to his store, intending on either selling them or ripping them apart for parts if necessary for his customers.
Elmo receives a man named "Zeke" that day, who requests a replacement blender motor for his wife. As a suave salesman, Elmo fibs to him that he has a new shipment of blenders, but in reality had only one from his working blender. Blender desperately tries to hide himself, but is discovered and taken part. in a scene bearing resemblance to a surgery scene in a horror film. Upon the removal of his motor, Blender apparently dies and Elmo sells off his next product. Zeke then returns and requests some radio tubes, also for his wife, but only with Radio being available to provide such parts. The gang tries to hide him, but he, too, is discovered. Lampy then forms a plan to save their friend.
Just as Elmo is preparing to dissasamble Radio, Toaster draws the curtains and plants himself on top of Kirby with Blanky on his head, approaching Elmo like a ghost. Elmo sees his reflection in Toaster's metal plating and screams in terror, running around the storage room until he smashes into a pole, knocking him out. The appliances then all escape to who-knows-where, wrecking Elmo's store in the process. By the time he comes to, he has no idea what had happened, but his customer pops in, still requesting the unavailable radio tubes. Elmo's business probably suffers as a result of the appliances' escape.
Elmo St. Peters shares some similarities with Sid Phillips from Toy Story (It is likely Sid is inspired from Elmo St. Peters):
They often rip objects apart or reassemble them in a bizarre way, making them a villain to said objects.
They own a lot of objects that are mutated as a result of being ripped apart or reassembled.
They end up being scared out of their wits by the objects who come to life in their presence, and both cases involve said objects rescuing a friend from suffering a terrible fate.
His name is a reference to St. Peter, the gatekeeper of Heaven. This is likely an allusion to the fact that both are symbolic of the afterlife, as Elmo's shop is the final resting place for some appliances.