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. Klein 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.