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Robert Swayze "Robby" Keene (Tanner Buchanan) is Johnny's estranged teenage son. In season 1, Robby drops out of school when he gets in trouble for drugs, after which he and his friends Cruz and Trey spend their days engaged in petty theft as seen when they are stealing laptops from Tech Town. Johnny tries to reconcile with him at the apartment he lives at with his single mother, but he rebuffs him. To spite his father for neglecting him in favor of Miguel, Robby begins working at LaRusso Auto Group. Despite his original ulterior motive, Robby grows to appreciate Daniel's faith in him and turns against Cruz and Trey when they try to rob the dealership by fighting them off with his newfound karate skills and a security camera. Robby becomes Daniel's Miyagi-Do karate student and flourishes under this tutelage. However, when Daniel finds out that Johnny and Robby are father and son after Johnny shoves Daniel and breaks one of his All-Valley trophies, Daniel furiously banishes Robby from his home and business as punishment for his deception. Undaunted by this rejection, Robby enters the All-Valley tournament with no affiliation and advances towards the finals, impressing Daniel. This, combined with Amanda's encouragement to forgive Robby and Robby's apology for hiding the fact that he is Johnny's son, convinces Daniel to reconcile with Robby, officially become Robby's sensei again and allow Robby to fight under the Miyagi-Do name during the final match against Miguel. Robby sustains a shoulder injury from Hawk's illegal attack during the semifinals and ultimately finishes second to Miguel, who exploits Robby's shoulder injury to win a close match. Impressed with Robby's performance and disgusted with his students' increasingly thuggish behavior, Johnny apologizes to Robby, who politely accepts it, implying that Robby still cares about Johnny somewhat despite their strained relationship. Regardless of this loss, Daniel praises Robby's performance and takes him to Mr. Miyagi's old rest house to explain his plans to restart the Miyagi-Do dojo with Robby as his senior protégé.

In Season 2, Robby moves in with the LaRussos after his mother Shannon abandons him and leaves with her boyfriend Rick to go on vacation to Mexico, and is evicted from their apartment due to unpaid bills. He and Sam bond closely while training and enter a short relationship, but the relationship is short-lived because she cheats on him with Miguel while they are attending Moon's party due to Sam finding out from Aisha that Miguel returned Miyagi's Medal of Honor to Robby, which had been stolen by Hawk. After Robby takes Sam to Johnny's apartment after getting drunk at a party, Daniel furiously accuses Robby of following the footsteps of his father and cuts ties with Johnny and Robby. With his mother in rehab and his relationship with Daniel tarnished, Robby is forced to live with Johnny, and the pair slowly start to reconcile. During the karate war between the Cobra Kai and Miyagi-Do students at school, Robby once again gets into a fight with Miguel, who taunts him by mocking his relationship problems with Johnny and Sam. After a lengthy fight, Miguel eventually traps Robby in an arm lock and nearly decides to break his arm to end the school brawl once and for all but decides to let go of him after remembering Johnny's advice of showing mercy. However, Robby, furious with Miguel for taunting him about his relationship problems with Johnny and Sam, having a better relationship with his estranged father Johnny than he ever did and taking advantage of Sam at Moon's party, furiously retaliates by kicking Miguel over the balcony railing, accidentally sending him falling off the second floor and crashing spine-first on a stair handrail, paralyzing him and rendering him unconscious. Shocked and horrified at the fact that he accidentally crippled Miguel, Robby flees the scene while Sam, Hawk, and a police officer check on an unconscious Miguel.

In season 3, Robby is subsequently expelled from West Valley High School for crippling Miguel at the end of the school brawl and goes into hiding as a wanted fugitive for several weeks, stealing a used Dodge Grand Caravan from the LaRusso dealership and living out of a homeless camp. After chasing down several leads, Daniel discovers that a devastated Robby is visiting his mother Shannon in rehab to seek comfort and support and arranges to have the police arrest him, believing that Robby will get a lighter sentence if he surrenders. As the police arrive and interrupt Daniel while he is trying to convince Robby to agree to the plan for a lighter sentence, Robby angrily accuses Daniel of betraying him and ends their friendship by coldly ordering Daniel not to visit him in juvenile hall. During his incarceration at Sylmar Juvenile Detention Center, Robby feels abandoned when Johnny misses a scheduled visit to provide moral support to Miguel's family during his surgery and subsequently cuts ties with Johnny and is bullied by an inmate, Shawn Payne. Kreese takes advantage of Johnny's neglect to visit Robby, and upon finding out that an inmate is bullying Robby, suggests that Robby adapts to the Cobra Kai mentality of striking first. Taking Kreese's lessons to heart, Robby gains the upper hand on Shawn, winning his respect after he admits to not snitching. Around mid-December, Robby is released from juvie, but rebuffs both Daniel and Johnny when they arrive to pick him up due to his resentment towards them. After discovering Sam and Miguel are together again at the Miyagi-Do dojo, Robby furiously accuses Sam of dumping him for Miguel after he was sentenced to juvenile hall. With nowhere else to go, Robby furiously ends his relationship with Samantha and goes to Cobra Kai, where Kreese agrees to take him in. Robby is initially reluctant to join Cobra Kai due to Hawk's resentment toward him for his actions against Miguel at the end of the school brawl, but Tory convinces him to stay with Cobra Kai. The two soon befriend each other and find common ground in each other's broken home lives as well as their own betrayals from Johnny, Sam, and Miguel. As the newest member of Cobra Kai, Robby proves himself to Kreese and the rest of the Cobra Kai students (except for Hawk) after stealing a snake from the zoo.

While Tory leads the Cobra Kais that night in an attack on the LaRusso house, Kreese keeps Robby behind at the dojo to train him. When Johnny shows up at the dojo to confront Kreese over the attack on Miguel, he is horrified to see Robby wearing a Cobra Kai gi. Robby offers Johnny to re-join Kreese in Cobra Kai, but Johnny rejects, and attacks Kreese. Robby steps in and defends Kreese and engages in a fight against Johnny after angrily venting on the past about how Johnny was never there for Robby when he needed him. Robby is then unintentionally knocked out by Johnny who reluctantly pushes him into a set of lockers after trying to reason with him to no avail. Daniel joins the fray, and he and Johnny defeat Kreese before Miguel and Sam intervene to talk them out of killing him. When Robby regains consciousness, he coldly orders Daniel, Johnny, Miguel, and Sam to leave before continuing his training with Kreese.

In season 4, he aids Kreese and Terry Silver with leading the Cobra Kais by training them in Miyagi-Do techniques so that they can better counter Miyagi-Do and Eagle Fang. He also starts a romantic relationship with Tory and recruits a young student named Kenny Payne, Shawn's younger brother whom he mentors due to him struggling to fit in at school and dealing with bullies. Robby confronts Johnny about the bullying and tells him if the bullying doesn't stop, he will be forced to put an end to it himself. During the tournament, Robby is forced to fight Kenny in the boys' quarterfinals, easily defeating him. In the boys' finals, after Miguel chooses not to return, Robby faces off against Hawk, who has returned to using his original name, Eli, and is allied with Miyagi-Do. As they both know the Cobra Kai and Miyagi-Do techniques, Robby and Eli fight to a draw, leading to a sudden death overtime. Ultimately, Eli defeats Robby and becomes the All-Boys champion after Robby hesitates to finish him off when Kenny cheers for him from the sidelines. Later, at the now-abandoned Cobra Kai dojo, Robby tearfully breaks down and admits he's tired of blaming Johnny for his problems and begins reconnecting with him, having become disturbed and horrified after witnessing Kenny pulverizing a helpless Anthony before his final match with Eli because of becoming corrupted by Cobra Kai. This makes him realize what his anger had turned him into as he failed to prevent Kenny from going down the same path.

In season 5, Robby joins his father in going to Mexico to find Miguel, which they succeed, but the two boys remain at odds with each other due to their past history. After Johnny reminisces with Daniel about how their own rivalry might've ended much earlier if they'd just fought it out, Johnny has Robby and Miguel fight it out with each other, ending with Miguel poised to send Robby over a railing, the same as Robby did to him. Miguel backs off and he and Robby finally make amends with each other and Johnny accidentally reveals to the two that Carmen is pregnant with their future half-sibling, much to the joy of both boys. Subsequently, Miguel and Robby become friends and Robby defects from Cobra Kai to rejoin Miyagi-Do in their fight against Cobra Kai. As a result, Tory breaks up with him while Kenny angrily ends their friendship. During the fight at the flagship Cobra Kai dojo, he reluctantly fights a vengeful Kenny who wins by using the "silver bullet" to incapacitate him but is saved by Miguel. After Silver is taken down, Robby tries to talk to a saddened Kenny, but the latter requests to be left alone and leaves in shame. He resumes his romantic relationship with Tory with a kiss.