He is the owner of the 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 SAO that Heathcliff's health has never gone into the yellow zone.

Spoiler

Heathcliff is the creator of Sword Art Online, Kayaba Akihiko, and the final boss on the hundredth floor. This is revealed on floor 75, where he and Kirito have a duel to decide the fate of the players.

(Source: SAO Wiki)