The protagonist of the series, Sword Art Online, Kirito is a "solo" player and has been branded a beater (beta tester and cheater) by the players in Sword Art Online. His game alias is created by taking the syllables of the first and last Kanji of his real last and first names respectively: (Kirigaya, Kazuto).

Asuna is the main female protagonist of the series, Sword Art Online, her game alias is the same as her real world name. Asuna is a friend of Kirito and is a sub-leader of the Knights of the Blood Oath, a medium-sized guild acknowledged as the strongest guild in Aincrad. Being one of the few girls that play SAO and extremely attractive, she receives many invitations and proposals. Asuna is a strong-willed, accomplished player known in-game as the "Flash" (閃光 Senkō) for her extraordinary sword skill.

Initially a side story character in SAO, Yui later becomes an accompanying/support character in ALO. At first thought to be a lost child in SAO, she is actually an artificial intelligence program. Her full designation is «Mental Health - Counselling Program», MHCP version 1, codename, «Yui». She was designed to monitor the emotions of players and appear at their sides to hear and help them out. However, since the moment everyone was imprisoned in Sword Art Online, she was forbidden to come in contact with the players and complete her prime directive. Unable to do anything but suffer as she monitored the emotions of the trapped players, she came upon Kirito and Asuna whose emotions stood out as a beacon of joy and peace in a sea of fear and despair. She sought them out, appearing to them as an amnesia child whom they adopted.