Geraldo has no last name because there are no identification documents on his person. The text tells us that Geraldo had nothing in his pockets at the time of his death. Because he had no identification papers, the hospital has no way of determining Geraldo's actual identity. The only reason the hospital knows his first name is because Marin provides it.
Marin also identifies Geraldo as a "wetback." This is a derogatory term referring to Geraldo's status as a Mexican illegal immigrant. The text also tells us that Geraldo was one of those men who always looked ashamed. It is likely that Geraldo kept a low profile because of his undocumented status. He may have feared being discovered by the authorities. Additionally, he spoke only Spanish and likely found it difficult to interact with the majority of people in his new surroundings. So, these are probably some of the reasons Geraldo never revealed his last name or specific details about his life to anyone.
From the text, we can conclude that Geraldo sometimes allowed himself to indulge in superficial conversations with others. For example, the only thing Marin knows about Geraldo after dancing with him is his first name and the fact that he worked in a restaurant. Geraldo's known reticence and lack of documentation means that he will never be identified. His family will also never know why he is not heard from again.
Geraldo's death was not inevitable. However, with no English-speaking or reading skills, Geraldo would have been at a loss when it came to deciphering street signs. He would have had to rely on guesswork to navigate the streets of the city. So, although his death was not inevitable, his life had been on difficult terms.
In The House on Mango Street, Esperanza describes a guy whom Marin meets one night at a party. Marin does not have a chance to get to know him; all she knows is that his name is Geraldo. Later that night, he is in a car accident and rushed to the hospital. Marin goes with him, but she cannot tell the doctors any details about him. On the literal level of the story, Geraldo does not have a last name because he only has a brief relationship with Marin at a local dance. However, on a figurative level, Geraldo is symbolic of many young men who leave their home countries to find better lives in America. Near the end of the vignette, there are references to men working any job they can get, living frugally, and sending money home to family. This is the reality for many people, and Geraldo is representative of this population. He, like many others, tries to find some dreams in the Mango Street area, but loses his dreams to harsh reality.