Mario is the pimp of the prostitute Eva Cortes. He resides in Casa Madrugada.
Marston encounters Mario severely beating Eva Cortes behind the Saloon in Casa Madrugada. Marston stands up for Eva telling Mario to stop beating her. Mario tells Marston that she is his property and he can do what he wants with her, but he will trade his "cow" for $200.
Marston 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 to check on Eva, but she is nowhere to be found. A nun tells him she left with a stranger (Mario) to Sepulcro.
When Marston arrives, 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 him before or after he gets his money back. Mario responds saying she is just a stinking whore. The two engage in a duel. Marston wins by shooting Mario several times, killing him and avenging Eva's death. Marston then retrieves his money from Mario's corpse and leaves.