Lengel is the stuffy manager of the A&P. He's also a local Sunday school teacher. He is the story's villain, the guy who embarrasses the girls and threatens Sammy with a horrible future. Lengel comes into the story late, just in time to stop all the fun from happening.
Although Lengel is the villain from Sammy's limited point of view, some readers might identify with him and even think he's right. Do you think it's okay for people to go grocery shopping in just their swimsuits?
Lengel, by the way, was the name of Updike's mother's boss at her job as a saleswoman.