- "I'm a Dreamer. My parents brought me here when I was two. I didn't even know I didn't have an American birth certificate until I was 17. My DACA status is up for renewal, and if I don't pass this test, I won't have a job or any employable skills. I don't know what I'll do if they kick me out. So yeah I'm feelin' the pressure."
- —Mateo Chavez[src]
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 debuts 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. 
In "Friends Like These", he studies for his academy exam at the firehouse because it's impossible to study with his roommates. Owen asks Marjan to help Mateo passes the exam because she has the highest score in the firehouse. Marjan tells Judd, TK and Paul that each of them have to read a part of the Austin FD Academy Manual so Mateo can listen to them. They also ask him questions when they're working or even resting. Marjan asks him questions from the test to help him. She tells him to not put so much pressure on himself but he tells her that he has to because if he fails this test, he'd get deported. He goes to pass the test but instead of Marjan being the one asking him questions, he comes face-to-face with his old captain but in the end, he got a 92%.
During "Hold the Line", Mateo works with TK and Buck while they're digging a ditch to help stop the fire. TK and Buck start comparing their rescues and while they're doing that, a car on fire starts falling down the hill and TK pushes Buck out of the way, saving his life. They all work together to unlock the car and a dog comes out of it, his owner running up to them, apologising about what happened. Then, TK and Buck keep bickering about their best rescues and Mateo stops them by saying it's an impossible debate because it's like comparing Iron Man and Captain America. They meet up with the rest and TK learns what about to his dad. He's there with the others to join TK and Buck in their quest to save Owen and Hen.
In "Difficult Conversations", Mateo gets a new tattoo on his shoulder blade that represents a firefighter helmet with the words and typo "Guardian Angle". Paul and TK are the ones to notice it first and don't know how to tell him. They wait until they're spending time in the training room and try to get Marjan to be the one who tell him about the typo. Later, in the bunk room, Marjan tells him about the typo and when Mateo says that he's so stupid, Marjan tells him that he's not, he's just dyslexic and that she'll find a way to fix his tattoo. After the tattoo appointment that Marjan manages to get for free after having a "conversation" with the tattoo artist, they replace the word "Angle" with flames and TK and Paul apologises for not telling him about it.
In "A Little Help From My Friends", Mateo is woke up by his roommates playing music, laughing and partying. He leaves his bed to tell them to quit it because he just came back from a 24 hours shift and need sleep. Few instant later, he realises that he can smells gas and tells everyone to leave the house quickly. Once outside, he tells the other that the house could've explode with all of them in it but tells them that as long as no open flame were still on, it should be fine. One of his roommate, then, tells him that she left some incense burning in her room and a few seconds later, the house explodes. Later in the week, Owen finds Mateo sleeping in the training room after they both got kicked out of the firehouse by the next shift. Mateo asks him why Owen isn't at home and Owen asks him the same. Then, Mateo reveals to Owen that his house blew up and he didn't tell anyone because he was ashamed of the fact that he's a firefighter and let his house exploded. At the end of their discussion, Owen says that it sounds like Mateo is looking for a new roommate and when Mateo confirms it, Owen tells him that he knows someone who's looking for one too, himself. During shift, Mateo joins Paul and Marjan and he's nauseous at the mention of food due to being hangover. He reveals to them that he has now become roommate with Owen and last night had been hell. There's a flashback from the last night where Mateo asks Owen if TK will be cool with him staying in his room and Owen reassures him about TK not living there anymore. Owen gets the tequilla out and they both start drinking, a lot. After a while, a drunk Owen tells Mateo that he didn't reschedule his cancer surgery, he cancelled it because he doesn't feel like it's the right time anymore. Mateo reveals it to Paul and Marjan and during a call, later in the day, they start talking about what happened with Owen but Owen interrupts them as they are talking about it on the open channel.
After the intervention TK staged for Owen, Mateo tells Owen that he's going to crash at Paul's place for the night because he really needs some sleep. In "Slow Burn", he's living with Owen again. In "The Big Heat", Mateo is at Owen's house with Buttercup when the police comes in with a warrant to search the premises. Later, at the firehouse, he's telling TK, Marjan, Paul and Judd about it. Later that day, Mateo is cleaning up the house before TK comes in and he tries to comfort him about Owen's arrest.
Marjan and Mateo are good friends. She's the one helping him the most when he has to pass his academy exam and who cheers him up when he's feeling down. When he learns about Marjan's engagement to Salim, he's sad because he doesn't want her to leave Austin at all.
TK and Mateo are friends. Mateo often acts like a little brother to TK. TK (and the rest of the firecrew) helped Mateo pass his exam by reading manuals to him via recording to make it easier for him. When TK comes to the station, Mateo hugs him, forgetting about his wound.
- 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.
- His favorite Marvel character is Thor. ("Difficult Conversations")
|"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"|
|"Back in the Saddle"||"2100°"||"Hold the Line"||"Friends With Benefits"||"Difficult Conversations"|
|"Everyone and Their Brother"||"Displaced"||"Bad Call"||"Saving Grace"||"A Little Help From My Friends"|
|"Slow Burn"||"The Big Heat"||"One Day"||"Dust to Dust"|