Lewis is one of Bart's friends and fellow fourth-graders in Mrs. Krabappel's class. He once annoyed a guilt-ridden Bart when he hid under a pile of leaves and pretended to be Principal Skinner's corpse, and he failed to vote for Bart in the class presidential election. He is most commonly seen playing in the schoolyard with his friend Richard.
Lewis was one of the children in the Model UN Club that were stranded on an island and, thinking he was going to die, Bart mistook him for Wendell. Lewis also plays on the Mighty Pigs pee-wee hockey team and according to gossip is a very good ballroom dancer. He apparently owns a gun as he was seen discharging it in school during the episode  as the class cheered after learning they were going to have a movie day.
In earlier seasons, Lewis could be considered as Bart's second-best friend. As the roles of Nelson and Milhouse were enlarged, Lewis was seen with Bart less and less often and was finally relegated to background character status.
His most recent speaking role is in "Stealing First Base", when after Edna Krabappel's class is moved to another classroom, he asks a student if he can sit with him. He was seen with Nina Skalka at the Le Petite Appetit during the episode, "The Daughter Also Rises". It's likely the two were dating during the episode and it's possible Nina is Lewis' girlfriend.
Lewis normally wears a tanned shirt with a single cream stripe, green shorts, and tanned shoes. He has black hair. In "Bart of Darkness", he was seen wearing orange swimming trunks. In the episode "Simpsons Roasting on an Open Fire", his hair color and skin complexion changed during one scene in the episode. He wears purple pajamas and purple slippers.
Lewis's full name ("Lewis Clark") was revealed on the episode Yellow Subterfuge, written on Skinner's list of students. It is possible that this name is a homage to Captain Meriwether Lewis and Second Lieutenant William Clark, two noted American explorers.