Marston encounters Mario severely beating Eva Cortes behind the Saloon in Casa Madrugada. Marston stands up for Eva, demanding Mario to stop beating her. Mario defends himself by saying she is his property, and he can do whatever he wants with her, but he will trade his "cow" for $200.
Marston then buys Eva's freedom. Eva gives Marston her gratitude and flees to Las Hermanas where she promises to become a nun. Marston visits the Church in Las Hermanas soon afterwards to check on Eva, but she is nowhere to be found. A nun informs him that she left with a "stranger" to Sepulcro.
When Marston arrives at Sepulcro, he finds Mario digging up a body in the cemetery, which turns out to be Eva; he had killed her in a drunken rage. Marston angrily asks if he should kill Mario before or after he gets his money back. Mario responds by saying that Eva was just a "stinking whore". The two engage in a duel, where Alcalde is gunned down and Eva is avenged. Marston then retrieves his money from Mario's corpse and leaves.
- Mario's character is very similar to Eduardo the Pimp, the primary antagonist in Cormac McCarthy's novel "Cities of the Plain".
- As shown in the stranger mission's quest description, Mario killed Eva while he was drunk. This explains why he says he didn't kill her, but rather hit her with too much force.
- He may be related to Santino Alcalde, a minor character found in Chuparosa.
- It is possible to kill him before the duel, but this will cause the mission to fail.
- Before approaching him in Sepulcro, his idle dialogue towards Marston will still be as if he was waiting for Marston to pay him. This is a flaw in the game's script.