Akihiko Kayaba/Heathcliff

Akihiko Kayaba is the overall main antagonist of Sword Art Online franchise. He is the development director and Game Master of the titular in-universe game Sword Art Online. He is also the designer of the NerveGear, the Cardinal System and The Seed, as well as the main antagonist of the Aincrad Arc. He plays SAO as the leader of the Knights of the Blood, Heathcliff.

In Kayaba's childhood, Kayaba expressed an interest in reading books. He was not particularly engrossed in games in computers, but he participated in the Mathematics Club of his high school. Later on, in university, he did not participate in any circles because he frequently worked in and out of laboratories.

When he began developing Sword Art Online, Kayaba toiled for years to develop the game software. In the anime, his main goal and dream was to "make Aincrad float" but after he did so, he wasn't entirely sure what his dreams, aspirations, or purpose were. In the original light novel, however, he explains more thoroughly that he felt frustrated and limited by the arbitrary rules that govern the current reality and wished to exceed them by supplanting it with his own. This desire went to the extent that he is actually happy to see Kirito and Asuna surpass the established laws of the virtual world he created.

It can be deduced that Kayaba began working on Sword Art Online before his third year in university. His love interest was Koujiro Rinko, who assisted Kayaba for ten years of the game's development, and took care of his body during the course of Sword Art Online.

His avatar, Heathcliff is the leader of what is considered to be the Sword Art Online's strongest guild Knights of the Blood. He is the owner of the first Unique Skill Holy Sword and is considered to be strongest player in-game. He wields the one-handed sword and a tower shield collectively called as Liberator. He also was able to hold off the boss of the fiftieth level for ten minutes by himself, which earned him the title Man of Legend. It is also rumored in Sword Art Online that Heathcliff's health has never gone into the yellow zone. This is due to his identity being Kayaba Akihiko and being able to make himself invincible or otherwise making his character into an Immortal Object after his health reaches 50%.

As a leader of the Knight of the Blood, Heathcliff was quite renown for his general tendency to stay out of guild affairs, preferring to allow his sub-leaders to develop plans and advance the game. Apart from guild paperwork and battles, Heathcliff rarely interfered with the choices and actions of his sub-leaders. The only exception to this was when Asuna expressed her wish to leave the guild, and Heathcliff refused to allow her do so without challenging Kirito to a duel.

Kirito first suspected his true identity during his match against Heathcliff when Kirito saw that Heathcliff's speed exceeded the system limits. However, Kirito figures out his true identity in the 75th floor. Thanks to a miracle in the game allowing Kirito to launch one final attack after his HP ran out, Kirito manages to defeat Heathcliff and free the trapped players from the cursed death game.

Even before the SAO incident occurred, Kayaba had decided to die along with  the game, though his choice of death was rather unusual. After SAO was cleared, he used a remodeled FullDive machine to perform a super-highpowered scan of his brain to transfer his consciousness into the virtual world permanently. This scan led to burning out his brain cells ultimately killing his body.

Although the scan only had less than a one in one thousand chance of succeeding, apparently the scan was successful, as he makes a reappearance in ALfheim Online. He notes that his program was only recently awakened after hearing Kirito's voice during his encounter with Oberon. When Kayaba sees Kirito giving up and submitting to the power of the system, he reminds the swordsman about their battle, where Kayaba was forced to acknowledge the power of the human will over machine, and encourages Kirito to stand and take hold of his sword. After Kirito stands up and grabs the fist of Oberon, who attempted to punch him, Kayaba reveals his administrator ID and password to Kirito, allowing him to access Heathcliff's account.  In terms of administrative power it over-ranked Oberon and allowed Kirito to easily defeat Oberon. After Oberon is destroyed and Asuna is logged out, Kayaba returns and entrusts Kirito with the World Seed, as a compensation for helping Kirito. Although Kayaba wishes to have the seed sprout, he leaves the choice of what to do with it to Kirito as he disappeared into thin air.

And he is reprised by Marc Diraison in the English dub and Koichi Yamadera in the Japanese dub as Akihiko Kayaba and as Heathcliff reprised by Kevin M. Connolly in the English dub and Toru Okawa in the Japanese dub.