Rosalita "Rosa" Diaz (portrayed by Stephanie Beatriz) is a main character in all eight seasons of the show. Previously a detective, she quit the force after the Black Lives Matter protests in 2020 (Season 8) and became a private investigator helping victims of police brutality. Despite leaving, Rosa still joins the squad for some events and occasionally assists them with cases when she deems it necessary.

She was considered the "tough guy" and the "scary one" while working at the 99, and most of the members of the precinct were frightened by her. Rosa is intimidating, smart, secretive, sharp-tongued, and difficult to read. Despite this behavior, she shows to care deeply about those she deems her friends and will protect them at all costs. In the episode 99, she reveals that she is bisexual to Charles, and officially comes out to everyone in Game Night.

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