(( Spoilers abound, soooo just a warning. It's mostly pasted from a wiki, because lazy. - 3- ))
Kazuto Kirigaya, known by the online alias "Kirito", is the hero of Sword Art Online, with many deeds done in his life that I refuse to summarize because it'd still take too damn long. If you're at all curious, go watch the show/read the books. Of course, your mileage may vary. ...Also, again, spoilers below so be careful if ya care at all.
Kirito is someone who is not good with words, as said by Klein, and can be quite direct. This is due to him not having many friends and closing others off, because of his love for computers. As such, he can come off as a rude person to others, such as Lisbeth and Sinon. Sometimes, he actually seems to enjoy angering and teasing people as well. Also, from time to time, Kirito can be quite overconfident thinking he can do things others can not.
He is almost always a calm and collected character, rarely showing any signs of distress. When Kayaba Akihiko appeared in SAO and announced that it has become a Death Game, Kirito showed only a slight reaction and was able to quickly accept the new conditions of the game, while many others panicked. He has shown different moods throughout the series; at times being rather cheerful, while being gloomy at others. When in a battle, he becomes incredibly fierce and aggressive. Sometimes, his fierceness is up to the point where he loses control of himself and is even unable to recall what he had done.
However, Kirito is actually a kind person who is just misunderstood. Despite the general population opinion towards Beta Testers in SAO, he refused to abandon another player in trouble. This is only strengthened after the Moonlit Black Cats's demise. He also comforts others who are scared or down such as Silica after she lost Pina. He claims to help some female players in SAO because they remind him of his sister, who he feels to have forced into kendo and feels guilty for that. These kind of acts have earned him respect and affection from many girls. Kirito, unlike usual male heroes, is not very dense as he was able to tell that Lisbeth was going to confess to him, before Asuna interrupted them.
Kirito is also a very emotional person and lets his emotions control him from time to time. The deaths of Sachi and Moonlit Black Cats had haunted him until his marriage with Asuna. Sachi's death in particular had a huge affect on his personality, and motives; making him very distant from others for a while. This worries his friends and allies as Kirito is someone who would let guilt ruin his life, if not comforted. Kirito temporarily loses his will to live when Asuna's character was killed by Heathcliff. He also tries to hide his abilities, not wanting unneeded attention.
A person with incredibly strong resolve, Kirito will do anything and everything in his power to accomplish something that's important to him, without any hesitation. When Asuna was being held in ALfheim, Kirito was bound and determined to set her free and, when Yui confirmed that she detected Asuna being at the top of Yggdrasil, he immediately tried to fly up to the tree, which had already been found to be an impossible task, and tried to fight his way to the top by himself.
After escaping SAO, Kirito attempts to repair his relationship with Suguha, his cousin. This shows Kirito is now attempting to make up for his past mistake of ignoring her, showing a sense of maturity. Kirito, later in the series, is shown to be a calm and mature player who can be teasing as well (something that annoys Sinon to no end, her being his main target).
Kirito comes in four flavors. Have a pick between:
