Tsuboi Ryoutarou

Klein (クライン) is one of the 10,000 players who were trapped in «Sword Art Online» and the guild leader of «Fuurinkazan» (風林火山), one of the clearer guilds. He became Kirito's friend after he asked Kirito to show him how to play SAO. His real name is Tsuboi Ryoutarou (壷井遼太郎).

Real Life / Sword Art Online Avatar
Klein dons a headband over his red hair (even IRL, it would seem), and his physique seems to be one, or perhaps two, cuts above the average child physique of the SAO universe. During the two years in Aincrad he uses red robes and black armor resembling those of a samurai including a katana, along with his guild members.

His facial features includes a thin chin strip, thin eyebrows, a sharp nose, and a pair of gentle eyes.

ALfheim Online Avatar
In ALfheim Online, Klein's appearance is mostly the same as his appearance in Sword Art Online (which is also his real-life appearance), except that his hair is dyed red in ALO. Klein plays as a Salamander in ALfheim.

Background
Originally, he was a new player who spotted Kirito as a beta-tester, and requested help from him. Kirito trained him the basics of the game and was impressed with his progress. Klein later rejoined his friends in order to form a guild. He was a stern supporter of Kirito despite the general gaming opinion of beta-testers.

Personality
Klein is one of the friendliest people in the series and sets the mood when he's around. He never holds grudges against others and is always ready to lend a helping hand. He cares deeply for his friends and always wants to be by their side fighting alongside them when they are in danger even to the point of getting angry when Kirito was fighting Death Gun, saying he should've converted to GGO as well. He gets angry when people get into situations that can get them killed since he wants to protect as many people as he can.

Klein is a very loyal person which is apparent in his friendship with Kirito. He is very loyal to Kirito despite Kirito usually treating him with coldness and him being a beta-tester. It is later revealed this coldness is due to Kirito feeling guilty for leaving Klein behind at the beginning of SAO. Klein does not care about this however and the two make up as Klein is shown to be the person Kirito relies on the most beside Asuna.

Klein is also a "bachelor" and is shown to try to get together with various women in the series, such as Asuna and Freya, and even Kirito's sister who he'd yet to meet. He usually has no luck however, making him somewhat of a comic relief.

Aincrad Arc
Early in the official version of the game, he notices Kirito as a beta tester, and asks to be taught the basics of fighting. Soon after understanding the basics, he attempts to log out so that he can eat his pizza, but notices that there was no log out option.

Afterwards, he and Kirito are recalled to the Starting City where Kayaba Akihiko announces that Sword Art Online has become a death game and a reality, shortly afterwards the avatars of player, including Klein's, are transformed to look like the real bodies of the players.

After the tutorial, Kirito takes him to the streets of the city, telling him that they needed to get to the next village together. But Klein refuses by saying that his friends must be in the plaza and that he needs to be with them. He suggests that they could all go together but notice that Kirito feels worried about the idea, so he dismisses it. Soon they parts ways, with Kirito heading to the next village and Klein going back to the plaza.

They encounter various times throughout the two years in Aincrad due to both of them being in the front lines later on in the game. The first known of such encounters was when Kirito went to the Wolf Plains to do some overnight leveling, as he spent most of the day helping his Guild, the members of the Moonlit Black Cats level up at a fast rate. Another meeting was a few days before Christmas, when they met at the Ant Hill on the 46th Floor, which was the most efficient leveling area at the time. Then again during Christmas, when he tries to convince Kirito not to battle Nicholas The Renegade alone, but he finaly lets him go while he and his Guild keep the Divine Dragon Alliance from taking on the boss. When Kirito comes back, he gives the item of revival to Klein, telling him that he must use it within 10 seconds after the person dying.

They are again reunited on the 74th Floor, along with Klein's guild, when they encounter Kirito and Asuna before the arriving of an Army platoon. He and Kirito later come to their aid against «The Gleam Eyes». They again meet up on the 75th Floor for the boss battle, and they fight «The Skull Reaper». After being paralyzed along with most of the front line players except Kirito, he tells the Black Swordsman that he won't forgive him for the first day until Kirito buys him a meal in the real world to make up.

Fairy Dance Arc
Klein is present at Andrew's Dicey Cafe to celebrate the «Aincrad Capture Commemorative Party», and was later also present during the unveiling of a content update on ALO, the «Floating Castle Aincrad», which introduced «New Aincrad» to the ALO universe.

Phantom Bullet Arc
Klein is seen watching Kirito battle during the «Bullet of Bullets» tournament at Asuna's rented a house at Yggdrasil City. Upon seeing «Death Gun», he reacts saying he knows this person from SAO as one of the members of the red guild, «Laughing Coffin».

Caliber
He is reborn as a Salamander katana-wielding warrior. He later helps Kirito obtain Holy Sword Excaliber by setting Freya, who later turns out to be Thor, free. Later when Skuld appears he asks for her address, to which his whole party is amazed when she actually gives him a contact address.

Mother's Rosario
He appears in New Aincrad's 27th floor labyrinth with Kirito and manages to stop the other guild for 3 minutes in order to allow Asuna and the Sleeping Knights to fight that floor's boss.

Sword Art Online

 * Level: 88
 * Main Equipment:
 * 「Karakurenai」 (Katana)

One Handed Curved Blade Skills

 * Reaver

Trivia

 * He was seen on the SAO release day along with the hardcore gamers who were waiting for 3 days in line.
 * His screen name, Klein was derived from “Kurain no Tsuboi” (Klein Bottle) based on his real name Tsuboi Ryoutarou.
 * He was one of the 10+ players who had the extra skill «Katana».
 * In the 4-koma Klein is much more prominent and he tends to bug Kirito with messages or calls.