A fictional character created by Robert A. Heinlein.
In the "Future History" timeline, Leslie LeCroix, an employee of Harriman Industries, was the first man on the moon. The main importance of this is that the Time Corps designates time lines by the first person to step on the moon in that timeline.
In addition to repeated references throughout the Future History series, Leslie LeCroix appears in "The Man Who Sold the Moon" and To Sail Beyond the Sunset.