Tsuboi Ryoutarou (壷井遼太郎, Tsuboi Ryōtarō), known as Klein (クライン, Kurain) in «Sword Art Online» (SAO), «ALfheim Online» (ALO) and «Project Alicization», is a supporting character in the Aincrad Arc and a reoccurring character through the Sword Art Online series. He was one of the 10,000 players who were trapped in Sword Art Online, where he was the guild leader of «Fuurinkazan», one of the clearer guilds. After Sword Art Online was cleared, he began playing ALfheim Online as a Salamander.
In both Sword Art Online and the real world, Klein wore a red bandana that held up his hair and was tied around his forehead. In Sword Art Online, he had spiky reddish hair, but he has brownish hair in the real world. He also had a hooked nose, a slight beard on his cheeks and chin, and, according to Kirito, he looked like a wandering samurai or a bandit. During the two years in Aincrad he used red robes and black armor resembling those of a samurai including a katana, along with his guild members.
In ALfheim Online, Klein's appearance is mostly the same as his appearance in Sword Art Online, except that his hair is dyed red in ALfheim Online.