Kagami was born in Japan. Her mother, Tomoe Tsurugi, is a world champion in fencing who also has a alliance with Gabriel Agreste, Adrien's father. She is enrolled in Collège Françoise Dupont in Paris, France.
In "Riposte", she came to the school's fencing tryouts to be enrolled onto the team and demanded Armand D'Argencourt to bring out his class's best fencer to prove her skill. Soon she was in a duel with Adrien but saw that they weren't getting any results with the machine, she proposed having a freestyle match. The two dueled so fiercely and quickly that they ended it the library. After dealing the final blow, Kagami asked Marinette who won, and she said Adrien did. She accepted her defeat and left but was approached by Adrien wanting to offer another match. However, Kagami told him her family doesn't give second chances and drove away.
She informed her mother of what happened over the phone, then was akumatized into Riposte. Upon finding Adrien, she accepted his offer, but he refused to fight in her current condition. She was intercepted by Ladybug, but Riposte found her as an inferior opponent. However, the heroine managed to escape with Adrien. Riposte found the pair among the rooftops where she calls out the rules of their match, despite Ladybug and Adrien trying to reason with her. Riposte said the damage was done and declared to the both of them that she was the best fencer which Adrien agreed too but wasn't convinced.
In "Frozer", she was seen ice-skating with Marinette, Adrien and Luka. She also gave Marinette some friendly advice of not hesitating.
In "Catalyst (Heroes' Day - Part 1)", Kagami was getting ready for Heroes' Day until Lila's deception as Volpina made her vulnerable to Scarlet Moth's akumatization, transforming into Scarlet Riposte. She joined with the rest of the mastermind's minions in meeting Scarlet Moth at the Eiffel Tower.
In "Mayura (Heroes' Day - Part 2)", she fought against the heroes during the battle and was briefly deakumatized but thanks to Dark Cupid and Scarlet Moth, Kagami was reakumatized again into her villain counterpart. It wasn't until the heroes deakumatized Hawk Moth that she and everyone was set free.
In "Animaestro", she was seen with Adrien and her mother, Tomoe. It was also revealed that she and Chloé Bourgeois hate each other due to the former's crush on Adrien.
In "Backwarder", she was boarding the train with Adrien as they were heading to London to attend a royal wedding, but before they left, she saw Marinette arrive and nervously give him a letter, finding his friend awkward. After Backwarder was defeated, she left for London and returned back to Paris on Monday.
In "Gamer 2.0", Kagami was practicing her fencing with Adrien until she and several other people were taken prisoner by Gamer 2.0. In her akumatized form, she was the titular villain's first avatar but was beaten by Ladybug as the Mime. After the heroes played through every character and defeated Gamer 2.0, she and everyone else was set free.
In "Oni-Chan", she was akumatized into the titular villain after seeing Lila kissing Adrien. As Oni-Chan, she tried to attack Lila in revenge.
In "Ikari Gozen", she had Marinette Dupain-Cheng as her partner. She was later given the Dragon Miraculous by Ladybug and transformed into Ryuko after her mother was akumatized into the titular villain.
In "Desperada", after meeting up with Adrien, the two were supposed to go practice their fencing but went to meet their friends at the Couffaine family houseboat instead. However, she mistakenly called Luka Marinette's boyfriend. Then, they heard a commotion above deck and saw Jagged Stone asking Anarka to join his band again, but Anarka refused. Soon things went from bad to worse when Vivica appeared as Desperada wanting revenge on Jagged. However, when he pointed out that Adrien was his new guitarist, the music villain turned her weapons on him. Furious, Kagami tried to defend him but was the first of villainess's victims, and it wasn't until Ladybug and Cat Noir and their new ally Viperion defeated the music villain that Kagami and everyone else were set free.
In "Feast", she was seen practicing her fencing skills in the park.
In "Felix", Kagami was on video chat with Marinette and the rest of their friends who were trying to figure out how they could make Adrien feel better on the anniversary of his mother going missing. They agreed with Marinette's idea of them sending a package of care videos.
In "Heart Hunter (The Battle of the Miraculous - Part 1)", she was seen with Marinette and Adrien as the three were having fun in Le Grand Paris. She was later recruited again by Ladybug to stop Heart Hunter.
In "Miracle Queen (The Battle of the Miraculous - Part 2)", she was seen as one of the royal guards of the titular villain. However, she did not transform as she had already given the Dragon Miraculous back to Ladybug.