Leonard Simms is a former U.S. naval officer who Hurley knew at Santa Rosa Mental Health Institute. Leonard was stationed in the South Pacific before and/or during 1988 and, along with his friend and fellow officer Sam Toomey, monitored longwave radio transmissions at a government listening post.
In 1988, the station picked up a transmission consisting of the numbers4815162342 repeating on a loop. (Danielle Rousseau and her science team also caught the transmission around that same time.) The Numbers stayed with Leonard and Sam. After Toomey used the numbers in a game of chance, a plague of bad luck seemed to befall those around him. Leonard and Sam believed the Numbers to be cursed, driving Sam to suicide, and Leonard to insanity. ("Numbers")
At an unknown time, Leonard was admitted to Santa Rosa Mental Health Institute. He spent all of his time playing the game Connect Four with himself, while endlessly repeating the Numbers. ("Numbers") ("Dave")
Leonard Simms experiences a moment of lucidity. ("Numbers")
After Hurley got out of Santa Rosa, he won the lottery by playing the numbers he had heard from Leonard, and came to believe they were cursed. Visiting Leonard at the institute to find answers, Hurley told him he used the Numbers in the lottery, to which Leonard broke from his near-catatonic state and became hysterical, screaming, "You've opened the box!" He told Hurley to get away from the Numbers or it wouldn't stop. Leonard then told Hurley that he heard the numbers from Sam Toomey in Kalgoorlie, Australia. ("Numbers")
Of the main characters, Leonard has met only Hurley, though it is highly likely he also met Libby.
Leonard only once stopped repeating the numbers (to the extent of our knowledge) when Hurley told him about the lottery prize. Leonard seemed to sober up, and he frantically warned Hurley of the dangers the numbers carried.
Leonard greatly resembles a bespectacled, balding patient Charlie Cheswick (played by Sydney Lassick) in the mental institution in One Flew Over the Cuckoo's Nest.