Lewis is one of the Guthrie siblings.
Lewis first appeared in the background in The New Mutants issue 42 (1986) and has appeared in the background of the Guthrie Family on many more occasions but was never truly defined until the X-Men arc known as "Endangered Species".
Major Story Arcs
Beast of the X-Men and Dark Beast came into contact with the Guthrie family in their home of Cumberland, Kentucky. It was during this meeting that Dark Beast witnessed Lewis displaying his "mutant powers". Lewis didn't in fact have mutant abilities, and some of his family (such as Melody and Jebediah) had actually lost their powers in the event known as M-Day, but Dark Beast saw potential in the youth and began experimenting on him. Dark Beast hoped to activate the latent X-Gene, which was surmised to be present in all of the Guthrie's. However, things did not go to plan and Lewis almost died as a result of the experiments - his body badly damaged. Lucinda Guthrie, his mother, discovered what had happened and shot at Dark Beast - chasing him from the premises.
Lewis Guthrie has not be seen since, but neither Paige or Sam have been shown to be mourning other than the loss of Joshua so it is safe to say that he survived the ordeal.