Francisco X. Stork didn't write Marcelo entirely from imagination. He's also a Mexican-American lawyer, but don't worry, he's more of a Jerry Garcia than an Arturo. (Source.)
You, too, can learn to sign "I love you" in American Sign Language, like Marcelo does to Aurora. It looks a bit like throwing the horns, for all you UT fans out there.
The character of Marcelo was inspired by the time the author spent living in a L'Arche house, which is part of an international community for people with intellectual disabilities. Want to do the same? Check 'em out.