This page contains information regarding Star Trek: Picard, and thus may contain spoilers.
Rios, a former Starfleet officer, rose to the post of XO on the USS ibn Majid, a heavy cruiser. Around 2389, Rios witnessed the violent death of the ibn Majid's captain, an event which prompted him to leave Starfleet. According to La Sirena's ENH, he had grown cynical and distrusting since his departure. (PIC: "The End is the Beginning")