Mari makes her first appearance when her brother Yuuri pays his respects to his deceased dog Vicchan at the household shrine. She welcomes him home, asking Yuuri what he plans to do now that he has graduated college and if he is going to continue skating or not. She says she is willing to support him if he does, though Yuuri says he hasn't decided yet. Mari leaves the subject at that and invites Yuuri to go have a soak in the hot springs.
Mari asks about all the luggage Victor has left in the hallway at Yu-topia, and later is shocked when a huge number of reporters and guests rush to the resort when they hear that Victor is currently staying there.
Later on, after Yuri Plisetsky arrives at Yu-topia, she becomes more than a little star-struck when she sees him due to his resemblance to her idol, Takao, from a boy band she follows. When Hiroko tells her that his name is also Yuri, like her brother, she says that's too confusing and she declares that Yuri's nickname will be "Yurio". Yuri isn't pleased with his new nickname, but from that point on, it sticks.
Mari and the rest of the Katsuki family, along with Victor, Minako and the Nishigori family, congratulate Yuuri for being assigned to the Grand Prix series. Axel, Lutz and Loop offer to explain how the Grand Prix series works to Mari and her parents, since they are unfamiliar with the figure skating world. Mari later gets excited when they mention that Yuri will be competing in the Rostelecom Cup.
Mari is seen briefly at the beginning of the episode with her parents, the Nishigori family and Minako, seeing off Yuuri and Victor before they leave for China.
Mari is seen with her parents and the Nishigori family at Yu-topia as they watch the free program section of the China Cup on television.
At the end of all the short programs, she calls Yuuri from the vet to tell him that Makkachin has gotten some buns stuck in his throat and that they don't know if he will make it.Otabek together in a cafe. Excited, they begin to chatter to each other, but stop when they are approached by Yuuri and Victor. They request to have dinner with all the skaters participating in the Final, and they get their wish (excluding JJ). In the end, they leave the restaurant with the others when Jean-Jacques arrives with Isabella.
She and Minako are shown before each of the skaters' performances (excluding JJ's) hold up flags and cheering them on.