Ryōtarō Tsuboi, known by his avatar name Klein, is one of the 10,000 players trapped in the death game Sword Art Online. He first meets Kirito on the game’s launch day, identifying him as a beta tester and asking for help learning basic combat. After the tutorial and Kayaba Akihiko’s announcement that logout is impossible and in-game death is permanent, Klein chooses not to travel with Kirito, deciding instead to find his friends from other games. This leads to the formation of his clearing guild, Fuurinkazan, which he commands. In Sword Art Online, Klein wears a red bandana over spiky reddish hair and has a hooked nose with a light beard along his cheeks and chin. His outfit consists of red robes and black samurai-style armor, and he carries a katana. His appearance in ALfheim Online remains similar, though his hair is dyed red and he wears standard black and red Salamander attire. Klein reaches level 88 in Sword Art Online and wields a one-handed katana named Karakurenai. His skills include One-Handed Curved Blade, Katana, Light Metal Equipment, Light Shield Equipment, Battle Healing, First Aid, Searching, Listening, Extended Weight Limit, Fighting Spirit, and Sewing. He is loyal and friendly, maintaining a strong friendship with Kirito despite occasional coldness. He often humorously pursues romantic interests with female characters and shows deep care for his friends, readily offering help and showing concern for their safety. During the Aincrad arc, Klein takes part in major events. He supports Kirito in the Christmas event by holding off the Divine Dragon Alliance so Kirito can fight Nicholas the Renegade, and later receives a revival item from him. Klein battles field bosses like the GeoCrawler on the 56th floor and joins the raid against the Laughing Coffin guild. On the 74th floor, he helps Kirito and Asuna rescue members of the Aincrad Liberation Force from The Gleam Eyes. He also fights in the 75th-floor boss battle against The Skull Reaper. After Heathcliff is revealed as Kayaba Akihiko, Klein is paralyzed along with other clearers. Kirito apologizes for leaving him on the first day, and Klein says he will only forgive him if treated to a meal in the real world. After Sword Art Online is cleared, Klein attends the conquest celebration at Dicey Cafe and the ALfheim Online afterparty. He joins Kirito’s group for the Deep Sea Plunderers quest. In the Phantom Bullet arc, he recognizes Death Gun as a former Laughing Coffin member while watching the Bullet of Bullets tournament. During the Calibur story, he aids Kirito’s party in clearing the Inverted Ice Pyramid, helps free the NPC Freyja—later revealed to be Thor—and receives Mjölnir, which he gives to Lisbeth. He also asks Skuld for her contact details. In Mother's Rosario, Klein helps Kirito support Asuna and the Sleeping Knights by delaying a rival guild in New Aincrad’s 27th-floor labyrinth. He later joins a barbecue, participates in a boss raid on the 28th floor, bids farewell to Yuuki in ALfheim Online, and attends a real-world picnic. In Ordinal Scale, Klein and his guild fight in augmented reality battles. He loses memories of Sword Art Online after an attack by Eiji, is hospitalized with a broken arm, and later aids in rescuing trapped SAO survivors.