Rios, a former Starfleet commander, rose to the post of executive officer on the USS ibn Majid, a heavy cruiser, where he served under Captain Alonzo Vandermeer. His serial number was SC-850-705. (PIC: "Broken Pieces")
In 2390, while patrolling the Vayt sector, the ibn Majid encountered a small ship of unknown origin, and brought aboard its two passengers, Beautiful Flower and Jana. The Captain received a "black flag directive" from Starfleet Security to kill the two beings, otherwise his ship would be destroyed with all hands. Rios confronted Vandermeer, after he murdered the two beings with his phaser, and the Captain shot himself in the mouth.
Rios covered everything up, just as Vandermeer was ordered to do. He beamed the bodies into space, deleted the transporter logs, and told the fleet that the Captain had committed suicide for no reason. Six months later, Rios was discharged from Starfleet, officially for post-traumatic dysphoria. (PIC: "Broken Pieces")
By 2399, Rios had become acquainted with Raffi Musiker, and had assumed command of the starship La Sirena, serving with his fully utilized Emergency Hologram basic installation, each of which was programmed in his own image. One was an Emergency Medical Hologram (EMH) which was programmed with a light English accent and named Emil. Another was an Emergency Navigational Hologram (ENH) which was programmed with a light Northern Dublin Irish accent and named Enoch. One was an Emergency Hospitality Hologram (EHH) which was programmed with a Midwestern American accent and aptly named Mister Hospitality. Another was an Emergency Tactical Hologram (ETH) which was programmed to speak Spanish and was named Emmet. Lastly, there was the Emergency Engineering Hologram (EEH) which was programmed with a Scottish accent and was named Ian.
When Rios was a child, his mother often sang him the lullaby "Arroz con leche". In 2399, he stated that he has no family. He also said that he liked to pretend that Vandermeer was his father. (PIC: "Broken Pieces")
He was an avid reader and kept a number of printed books in his quarters and lying around the ship. In addition to having read one of Jean-Luc Picard's books, he was fond of existentialist philosophy, having read philosophers such as Miguel de Unamuno, which he described as concerning "the existential pain of living with the consciousness of death and how it defines us as Human beings." (PIC: "The End is the Beginning", "Absolute Candor", "The Impossible Box")
Cristóbal Rios was played by Santiago Cabrera.