Miguel Diaz (Xolo Maridueña) is an Ecuadorian American teenage asthmatic with low self-esteem and is Johnny's neighbor. While shopping for Pepto-Bismol for his grandmother at a strip mall in Reseda, he is attacked by a group of rich bullies led by Kyler after accidentally foiling their plans to buy beer. The bullies nickname him "Rhea" after noticing the bottle of Pepto-Bismol and dump it all over him. When the bullies shove him onto Johnny's car, Johnny attacks them, beating them senseless before he is arrested. Miguel becomes Johnny's first student in the new Cobra Kai dojo. Aside from learning karate, he is the dojo's housekeeper and website administrator. Though initially reluctant and mocking towards him, Johnny quickly comes to care for Miguel, becoming a father figure to him, despite objections from his mother.
When Daniel's daughter Sam confronts Kyler over vicious rumors he spread about her after she fought off his attempt to date rape her, Miguel beats up Kyler and his clique, gaining popularity in the school and earning Johnny's gi as the fight leads to an influx of new members to Cobra Kai. Despite Johnny's warning that he should avoid the LaRussos, Miguel begins dating Sam. Unfortunately, they break up after four months of dating when Miguel wrongly suspects Sam of cheating on him with Johnny's son, Robby, leading to an altercation in which Miguel accidentally punches Sam. Cobra Kai's thuggish philosophy begins to affect Miguel's values and he becomes much more violent and ruthless, to Johnny's eventual disapproval, especially when Miguel fights dirty against Robby in the All-Valley.
In Season 2, friction develops between Johnny and Miguel when Miguel finds out the truth about Robby's parentage. Johnny makes amends with his star pupil by telling him the story of Robby's birth and how he was never there for him to his regret. He promises to do better by Miguel than he did with Robby. Miguel is also wary of Kreese, noticing inconsistencies in Kreese's stories about his supposed military exploits during the '80s and '90s. He vehemently disapproves of Kreese's style of teaching, and when he finds out that Hawk's gang trashed Miyagi-Do and stole Mr. Miyagi's Medal of Honor, he defeats Hawk in a brutal fight during a competition at Coyote Creek and returns the medal to Robby at Daniel's home, but not before appalling Johnny with his ruthlessness (which Johnny realizes is a result of Kreese's teaching).
Miguel develops a relationship with new Cobra Kai student Tory Nichols, but he still has feelings for Sam, culminating in him kissing her at a party after she learns from Aisha the truth about the medal (which Robby had withheld from her). The act is seen by Tory, who challenges Sam to a fight in school the next morning. In the ensuing rumble, Miguel once again battles Robby and even taunts Robby by mocking his relationship problems with Johnny and Sam. After a lengthy fight, Miguel gets Robby in an arm lock and nearly decides to break his arm, but chooses to show him mercy after remembering a conversation where Johnny expressed regrets over not learning to show mercy at a younger age. However, Robby, furious at Miguel for having a better relationship with his father Johnny than he ever did and seemingly taking advantage of Sam, furiously retaliates by kicking Miguel over the balcony railing and accidentally sends him falling off the second floor. Miguel hits his back on the stair railing as he lands, paralyzing him and leaving him in a coma.
In Season 3, when Miguel wakes up from his coma, he is initially resentful to Johnny for his predicament about showing mercy, but slowly reconciles with him when the latter becomes personally involved in his physical rehabilitation. Several people pitch in to pay for Miguel's surgery, Johnny by stealing and pawning a modern art sculpture from his stepfather, Bobby using proceeds from his church, and Sam by organizing a car wash fundraiser, although the Cobra Kais decide to initiate conflict with the Miyagi-Dos by stealing the proceeds to donate in their own name.
Under Johnny's mentorship, Miguel is able to gain full function of his legs. As Johnny has been kicked out of Cobra Kai by Kreese in the wake of the school brawl, he and Miguel open a new martial arts dojo, Eagle Fang, with students Kreese had ejected from Cobra Kai. He also cuts his ties with his Cobra Kai friends after finding out how Hawk broke Demetri's arm during the laser tag fight, and also learns that Kyler has joined Cobra Kai. This act wins over Sam, who resumes her romance with Miguel and gets her father to connect with him when he catches them making out in the Miyagi dojo. Daniel is surprised to find common ground with Miguel over their shared upbringings (coming from a poor, single-parent family, being bullied by wealthier kids, and finding guidance through karate) and agrees to let them continue dating. Miguel also works with Sam to save the All Valley tournament, and aids her in creating an alliance between Eagle Fang and Miyagi-Do, helping them fight off the Cobra Kais when Tory leads them in an assault on the LaRusso residence. In the fight, Miguel is singled out for a one-on-one fight with Kyler, who seeks payback for the lunchroom fight. Kyler brutally beats Miguel, but Miguel gains a second wind when he remembers all of Johnny's encouragement, and quickly defeats Kyler with a takedown counterattack.
Miguel returns to his mother's apartment to tend to his wounds from the fight, which is how Johnny finds him when he shows up at the Diaz apartment seeking to declare his love to Carmen. An enraged Johnny immediately sets out to Cobra Kai to confront Kreese, as does Daniel. Miguel and Sam manage to chase them down, showing up at Cobra Kai just as Daniel is about to finish off Kreese, bringing him and Johnny to their senses.
During the aftermath, Johnny and Miguel bring their fellow students to the Miyagi dojo to help train both dojo's students.
When Daniel's daughter Sam confronts Kyler over vicious rumors he spread about her after she fought off his attempt to date rape her, Miguel beats up Kyler and his clique, gaining popularity in the school and earning Johnny's gi as the fight leads to an influx of new members to Cobra Kai. Despite Johnny's warning that he should avoid the LaRussos, Miguel begins dating Sam. Unfortunately, they break up after four months of dating when Miguel wrongly suspects Sam of cheating on him with Johnny's son, Robby, leading to an altercation in which Miguel accidentally punches Sam. Cobra Kai's thuggish philosophy begins to affect Miguel's values and he becomes much more violent and ruthless, to Johnny's eventual disapproval, especially when Miguel fights dirty against Robby in the All-Valley.
In Season 2, friction develops between Johnny and Miguel when Miguel finds out the truth about Robby's parentage. Johnny makes amends with his star pupil by telling him the story of Robby's birth and how he was never there for him to his regret. He promises to do better by Miguel than he did with Robby. Miguel is also wary of Kreese, noticing inconsistencies in Kreese's stories about his supposed military exploits during the '80s and '90s. He vehemently disapproves of Kreese's style of teaching, and when he finds out that Hawk's gang trashed Miyagi-Do and stole Mr. Miyagi's Medal of Honor, he defeats Hawk in a brutal fight during a competition at Coyote Creek and returns the medal to Robby at Daniel's home, but not before appalling Johnny with his ruthlessness (which Johnny realizes is a result of Kreese's teaching).
Miguel develops a relationship with new Cobra Kai student Tory Nichols, but he still has feelings for Sam, culminating in him kissing her at a party after she learns from Aisha the truth about the medal (which Robby had withheld from her). The act is seen by Tory, who challenges Sam to a fight in school the next morning. In the ensuing rumble, Miguel once again battles Robby and even taunts Robby by mocking his relationship problems with Johnny and Sam. After a lengthy fight, Miguel gets Robby in an arm lock and nearly decides to break his arm, but chooses to show him mercy after remembering a conversation where Johnny expressed regrets over not learning to show mercy at a younger age. However, Robby, furious at Miguel for having a better relationship with his father Johnny than he ever did and seemingly taking advantage of Sam, furiously retaliates by kicking Miguel over the balcony railing and accidentally sends him falling off the second floor. Miguel hits his back on the stair railing as he lands, paralyzing him and leaving him in a coma.
In Season 3, when Miguel wakes up from his coma, he is initially resentful to Johnny for his predicament about showing mercy, but slowly reconciles with him when the latter becomes personally involved in his physical rehabilitation. Several people pitch in to pay for Miguel's surgery, Johnny by stealing and pawning a modern art sculpture from his stepfather, Bobby using proceeds from his church, and Sam by organizing a car wash fundraiser, although the Cobra Kais decide to initiate conflict with the Miyagi-Dos by stealing the proceeds to donate in their own name.
Under Johnny's mentorship, Miguel is able to gain full function of his legs. As Johnny has been kicked out of Cobra Kai by Kreese in the wake of the school brawl, he and Miguel open a new martial arts dojo, Eagle Fang, with students Kreese had ejected from Cobra Kai. He also cuts his ties with his Cobra Kai friends after finding out how Hawk broke Demetri's arm during the laser tag fight, and also learns that Kyler has joined Cobra Kai. This act wins over Sam, who resumes her romance with Miguel and gets her father to connect with him when he catches them making out in the Miyagi dojo. Daniel is surprised to find common ground with Miguel over their shared upbringings (coming from a poor, single-parent family, being bullied by wealthier kids, and finding guidance through karate) and agrees to let them continue dating. Miguel also works with Sam to save the All Valley tournament, and aids her in creating an alliance between Eagle Fang and Miyagi-Do, helping them fight off the Cobra Kais when Tory leads them in an assault on the LaRusso residence. In the fight, Miguel is singled out for a one-on-one fight with Kyler, who seeks payback for the lunchroom fight. Kyler brutally beats Miguel, but Miguel gains a second wind when he remembers all of Johnny's encouragement, and quickly defeats Kyler with a takedown counterattack.
Miguel returns to his mother's apartment to tend to his wounds from the fight, which is how Johnny finds him when he shows up at the Diaz apartment seeking to declare his love to Carmen. An enraged Johnny immediately sets out to Cobra Kai to confront Kreese, as does Daniel. Miguel and Sam manage to chase them down, showing up at Cobra Kai just as Daniel is about to finish off Kreese, bringing him and Johnny to their senses.
During the aftermath, Johnny and Miguel bring their fellow students to the Miyagi dojo to help train both dojo's students.