Geraldo is only mentioned in the chapter titled "Geraldo No Last Name." Marin meets him at a dance and is the last person to see him before he is struck and killed by a car. Here's what we know from Marin through Esperanza:
[Marin] met him at a dance. Pretty too, and young. Said he worked in a restaurant, but she can't remember which one. Geraldo. That's all. Green pants and Saturday shirt. Geraldo. That's what he told her.
From this description, Geraldo was just another guy Marin met when she went out dancing. She knew nothing more, nothing less. He was just a dance partner that suffered a tragic fate.
However, as the chapter progresses, Esperanza adds more to the story.
[He was] just another brazer who didn't speak English. Just another wetback. You know the kind ... They never saw the kitchenettes. They never knew about the two-room flats and sleeping rooms he rented, the weekly money orders sent home, the currency exchange ... His name was Geraldo. And his home was in another country. The ones left behind are far away, will wonder, shrug, remember.
Esperanza doesn't know any more than Marin does about Geraldo, but she provides this information to the reader. It seems Esperanza, and more-so Cisneros, is using Geraldo as a symbol for other immigrants who have come and gone, who are unknown, and who did not get the help they needed to succeed, live, and thrive. If this information is accurate, Geraldo is a hardworking man who is trying to provide for a family back home. He came to the US to make money and send what he earned back home only keeping enough for himself to eat, sleep, and go out dancing on Saturday nights.