Carlos Ramirez first appears in Summer Knight, though not by name. We first meet him in Dead Beat, and he appears again in Proven Guilty. He is one of four regional commanders of the Wardens and in charge of the western United States. He was personally appointed by the Merlin, and he has a talent for veils. He's in his early-to-mid twenties (old enough to drink in DB, but younger than Billy and the Alphas), making him the youngest wizard to attain this position. He was still an apprentice during Summer Knight and was present at Harry's trial.
Ramirez has a healthy disregard for authority that was largely inspired by observing Harry at council meetings. During Molly's trial, when his experience is brought into question by the Merlin, he delivers a verbal bitchslap worthy of Harry. As Luccio explains to Harry, for Ramirez and some of the other young wizards, he is "a symbol of defiance to the conservative elements of the Council, and a hero who will risk his life when his principles demand it." Harry calls Ramirez the "poster boy" for the new generation of wizards.
From "Dead Beat", by Jim Butcher:
"He had naturally tanned skin, dark hair, dark eyes, and sharp-edged, classically Spanish features. I remembered him in an apprentice's brown robe back then, and covering his mouth with one hand to conceal a grin inspired by some of my dialogue with the Council's bigwigs.
The brown robe was gone, and he looked like he had filled in a little since I'd first seen him, but good Lord, he was younger than Billy the werewolf. He wore a grey cloak that looked reasonably clean and not at all damaged, and black fatigues beneath that. A simple, straight sword hung from one hip, and was balanced on the other side by a holstered Glock and, I kid you not, three round fragmentation grenades. His staff was fairly new-looking, but there were enough dents and nicks in it to make me think he had kept things from hitting him with it, and he walked with a kind of arrogant confidence you see only in people who have not yet realized their own mortality."
Dead Beat - Harry and Ramirez team up to take down the Necromancers and end the Darkhallow. Harry earns the right to call him "Carlos" after letting Ramirez ride his dinosaur.
Proven Guilty - We learn that Ramirez is pretty much the only Warden Harry interacts with, post-Dead Beat. This makes sense, as one of the roles of the regional commanders is to coordinate with each other. At Molly's trial, he is charged with security. Ramirez presents evidence toward Molly's defense which may border on perjury: he is much more optimistic about Rosie and Nelson's recovery than Harry was earlier on in the book.
White Night - We find that Harry and Ramirez have been occasionally teamed up to continue training Warden Luccio's boot camp trainees. In their last training session, Ramirez got to witness Harry's ugly side in a deadly confrontation with ghouls in New Mexico. However he remains fiercely loyal to Harry, and provides backup in a showdown with the villains of White Night.