- "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 firefighter in 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 woken 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 an 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. At night, Mateo is cleaning up the house before TK comes in and he tries to comfort him about Owen's arrest. After they lost their house, Carlos and TK start living with Owen and Mateo as well.
In "Dust to Dust", the 126 is separated in different firehouses and Mateo ends up working alone at the 129 where the other men are bigots and treats him extremely badly. The captain refuses to let him come on a call with them and tells him that Mateo has to go pick up their lunches. Mateo goes out to pick them up and ends up being stuck in the middle of the dust storm, when it disappears, he sees passengers of a plane that crashed and starts treating as many people as he can, saving the lives of many. Soon he learns that some people are stuck in an elevator on top of a building and he runs up there, trying to save the people stuck inside, he's joined by Owen and the rest of the 126 who just arrived and together, they save the people stuck in the elevator. Once they're outside, the 129 and its captain arrive too and they start making fun of Mateo and telling him that he shouldn't be out there, Mateo snaps at them right before the captain passes out and is saved by Tommy, Nancy and TK.
Once the 126 and co. are at the firehouse to rebuild it, Owen announces to Mateo and the rest that when the firehouse reopens, Mateo won't be a probie anymore, he'll be an actual firefighter. He's also present when Billy tells them that he's shutting down the 126.
In "The Big Chill", Mateo is still with the 129 but the situation seems to be better for him there. Captain Tatum calls him his best guy during a call with Tommy, TK and Nancy where Mateo helps them save a man with a huge part of glass stuck on his throat.
The 129 joins the 122 at the collapsed and on fire shelter where Mateo sees Paul and Judd. Captain Tatum tells his team to gear up, find a hydrant that hasn't frozen yet and stop the fire. They put out the fire and create an ice palace. Carlos runs to Judd, Paul and Mateo to tell them that someone is still stuck inside, a teenager volunteer named Lindsey. Paul tells him that they had a list and Carlos replies that she was the one in charge of the list so she didn't count herself. Captain Andrews tells them that if they send a team in, they could be looking at a secondary collapse before Captain Tatum says they'd have to assume she survived the first collapse. Tatum tells Mateo that if he hears any sounds of the roof cracking, he has to get out of there and Andrews tells his team that if anyone isn't confident going inside, they can stay outside with no judgement. Once inside, Judd, Mateo and Paul and their teams are looking for Lindsey and they separate to have more space to explore. Judd calls out for her and they hear banging from Lindsey as a response. While Paul is separated from the rest, a part of the roof collapses and projects him away from where he was standing.
In "Thin Ice", Mateo is with Judd when he manages to convince Captain Andrews to let them go into the building to go save Paul and Lindsey. Captain Tatum stops him and tells Mateo that the 129 has another call they need to assist. He tells Mateo that their house have to move, and Mateo replies that while the 129 is house, Paul is his family and he wants to stay there and help save him. Tatum tells him that Mateo may not be the brightest kid but he's a good one and since Andrews accepts that Mateo joins his house for the rescue, he doesn't have a problem with Mateo staying.
Inside the building, Judd leads the team composed of Mateo and some members of the 122. He tells them to be safe and to not compromise the structure. They scream for Paul and tries to find him through their thermal detector without success. Mateo says that maybe they're going in the wrong direction, if they even know which direction is the right one. Judd tells him that they don't but he knows someone that could help them figure it out. Judd calls Grace through dispatch and tells her that the last time they saw Paul was at the south wall while Grace is looking at the building schematics of the structure. Grace realises that they're somewhere on the basketball court and Judd confirms it when he finds a hoop, Mateo telling him that "his lady is good" and Judd replying that it's why he called her. Judd asks Grace how they should proceed to find Paul and she tells him that their best bet is to "double back through the Alpha Bravo wall" because there's a lot of load-bearing beams and a good chance of air pockets for them to make it through. Judd thanks her and they're interrupted by Paul's voice on the radio, Mateo asks him where he is and Paul replies that he's in some sort of locker room, Grace tells them that the locker room is 25 yards northeast to where they are right now.
Judd and Mateo get out of the structure and join Captain Andrews outside again because they couldn't get to them from the inside. Andrews asks them what they suggest and Judd says that they'll try to make it through the outside with a couple of pneumatic jackhammers before opening a hole in a wall and make it to the locker room. Andrews tells them that vibration could make the building collapse even more, Judd agrees that it's a risk and Mateo says that Paul and Lindsey won't make it out of here if they don't try. Andrews tells them that he gets the feeling that even if he says no, they'll do it anyway and Judd tells him he's right. Judd radio-in Paul and tells him they're about to make a hole in the wall so it might get a little shaky where they are. After minutes, they manage to make a hole and Mateo and Judd go through it, helping Lindsey and Paul out of here.
Mateo is a person who always see the best in everyone and who is really easy to love but knows how to fight back when someone disrespects him. He works hard and never complains even when he's tasked with small missions at the firehouse. He's funny and often make his friends laugh by his actions and words.
Mateo and Marjan are close 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. Marjan is shown to be very protective over Mateo, for example when she learns that the tattoo artist and the people who were there made fun of him when the tattoo he thought of had a typo on it and she went to the tattoo shop with him to get them to redo it, for free.
Mateo and TK 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. Mateo is the one to comfort TK in "The Big Heat" when the latter comes to Owen's place after Owen gets arrested and his fight with Carlos. TK and Mateo now live together along with Carlos and Owen since their house burned down.
Mateo and Paul are close friends. Paul is shown to be protective over Mateo like a older brother would be, for example when he learned that the man making Mateo pass his writing exam made him look like a complete idiot. He's also teasing him every once in a while.
- 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. The resemblance between the two characters was explained in Season 2 episode Hold the Line, when Mateo asked Buck to check in on his LA based cousin, Marvin.
- He's allergic to cats. ("Yee-Haw")
- His favorite Marvel character is Thor. ("Difficult Conversations")
- After his house exploded in "A Little Help From My Friends", he became roommates with Owen and now lives at 1234 Iron Mountain DR.
|"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"|
|"The Big Chill"||"Thin Ice"||"Shock and Thaw"||"Push"||"Child Care"|
|"Red vs. Blue"||"In the Unlikely Event of an Emergency"||"3x08"||"3x09"||"3x10"|