Leon runs Shorepoint Base and is a friend of Alyx Vance and her father Eli who have both stayed at his base at some point in the past, when they left him the Scout Car they "put together".
When Gordon Freeman approaches his base after leaving Ravenholm at last, Leon is leading a squad against a Combine patrol who are assaulting Shorepoint. Winston, one of the soldiers under Leon's command, is wounded in the skirmish.
Once the fighting is over, Leon contacts Alyx by radio to inform her that Gordon made it through Ravenholm alive, then supplies Gordon with some ammunition and medical supplies, and directs Noriko to get the Scout Car ready for Gordon to use on his trip to Nova Prospekt. Leon is last seen trying to contact New Little Odessa to inform them of Gordon's arrival, with little success.