He is voiced by Hirata Hiroaki in the Japanese Version and Kirk Thornton in the English Version.
He is friendly player that meets Kirito at the start of the game, who quickly befriends and learns the basics from Kirito.
He and Kirito go their separate ways shortly after when Kirito leaves for another town to power level, while Klein remains behind to help out his less experienced friends, although he is concerned for Kirito due to his solitude and recklessness. Klein uses a katana to fight and later becomes a leader of his own guild, Fuurinkazan. He regularly meets with Kirito and his friends and occasionally joins them in quests in ALO, especially after the release of the World Seed.
- In an interview with Sword Art Online author Reki Kawahara, he noted that Agil and Klein are both "very popular", and he wanted to write short stories centred around their characters.
- His Japanese Voice Actor Hirata Hiroaki, best known voices as Leomon from Digimon Adventure and Gennai as Young from Digimon Adventure series.
- His English Voice Actor Kirk Thornton, best known voices as Gabumon and his Digivolution form from Digimon Adventure series, Shiro Fujimoto from Blue Exorcist, Shukaku from Naruto/Naruto Shippuden and Homer Yushima from Digimon Data Squad.
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Mother's Rosario Arc