Perez was a corpral his whole military career because he sucks balls. He's the guy who tells you to follow him to the guy on the roof in Halo 2. He is completely useless simply becuase he doesn't need to show you where his boss is, the building is straight forward. After he shows you where his boss is he becomes a background characther b/c no one likes him.
He went into the library with Johnson and Miranda, only b/c he was hoping he could catch keyes alone and get funky with her. Johnson, however, knew what he was up to and put him on door gaurd duty with McKenzie, who was gay. McKenzie was very muscular and easily overpowered Perez and began raping him, causing them not to hear Johnson when he told them to sound off. Tartarus walked in on them on his way to capture the two humans and fell in love with the muscular marine. Tartarus pryed McKenzie off of Perez who was crying and began raping McKenzie, killing him instantly. Perez tried to escape but was killed by AIDS. This event marked one of the gayest moments of the war.