Mateo Chavez is a character on FOX's 9-1-1: Lone Star who is a rookie firefighter in the newly rebuilt Station 126 located in Austin, Texas. He débuts in the first episode of the first season of 9-1-1: Lone Star and is portrayed by starring cast member Julian Works.
When he was two years old, Mateo was brought to the United States by his parents as an immigrant. He did not find out that he didn't have an American birth certificate until he was 17 years old. As a result, he enrolled in the DACA program in order to be able to work in the country. 
Joining the 126
While in the fire academy, he set multiple AFD academy records in multiple categories in field stuff. However, he failed the written test four times, but remained regardless, knowing he was born to do the job. He was referred to as a "frequent flyer" and was hired by Deputy Fire Chief Alden Radford to be his driver. One morning, when the Deputy came in with paper applicants for the new 126 crew, Owen Strand notices that Mateo is cleaning the inside of the door handles of the car, sparking his interest and requests for an interview to hire him. Chavez is initially confused that Strand had picked him to get an interview, but he assures he is correct. When asked why he was cleaning them since no one would know if they were dirty, Mateo responds with that he would know. Mateo tells him that all his teachers growing up had told him he was a little slow, but Strand reassures him that he isn't stupid and that he thinks that he is a thorough, relentless person, exactly who was needed for the 126. Shortly after he is hired, Owen had him tested for severe dyslexia, which proves to be true.
Working in the 126
Beginning his job as a firefighter as a rookie, Mateo is given the nickname, "Probie" by the rest of the 126. He is often put in smaller tasks away from the action during rescues, such as being a lookout during a factory explosion, holding pets, or pulling down the flag outside the firehouse. Despite occasionally feeling held back, he takes most of the tasks in stride and rarely complains about them.
A rescue where that shakes him, however, is at a house where he is tasked with just passing out water bottles and blankets for victims during the tornadoes. He watches the rescue and as Owen successfully retrieves the two children, their father who had been waiting for them suddenly passes out and dies from his injuries, having not told first responders earlier on he was bleeding internally. Mateo is notably shaken, having felt helpless through the rescue, feeling that he could have helped the man and thinking of the kids. Strand later consoles him, telling him not to beat himself up over it because it wasn't under his control. 
- Julian Works, who portrays him, previously appeared in the original series as troublemaker Marvin, who was last seen being taken away by Athena Grant in 7.1 to prison for carjacking and fleeing from police, but she promised to write a letter to the judge to shorten his sentence as a result of him saving a worker from a car wreck caused by the major earthquake in LA.
|"Pilot (9-1-1: Lone Star)"||"Yee-Haw"||"Texas Proud"||"Act of God"||"Studs"|
|"Friends Like These"||"Bum Steer"||"Monster Inside"||"Awakening"||"Austin, We Have a Problem"|