Just as Megan is like the daughter Rufus Sixsmith never had, Javier is like the son Luisa never had. Her eleven-year-old neighbor jumps the balconies between their apartments and lets himself in whenever things get too heated at home with his mom's boyfriend du jour.
We don't get to know Javier too well. He mainly serves as a humanizing force for Luisa, who might otherwise seem a little cold and totally focused on her work. Javier and Luisa also have an interesting conversation about fate vs. free will (which you can get a peek at in our "Themes" section on "Fate and Free Will"). Fay Li also threatens Javier's life to get at Luisa, but that reflects more on what a conniving villain she is than anything else.
In the end, Javier gets to move to San Francisco. His mom finally gets boyfriend who treats him well, and he gets to spend his days collecting stamps. That's another thing about popular mystery fiction, which Half-Lives is—it often has a happy ending.