Diavel

Diabel (ディアベル) was one of the 10,000 players who were trapped in «Sword Art Online» and is one of the few known beta testers. He appears as the leader of the 1st Floor boss raid party.

Appearance
Diabel had a bronze armor set on his chest, shoulders, arms, and shins, with a large longsword on his left waist, and a kite shield on his back. He had blue hair with long bangs and blue eyes. His clothes under the armor were also blue.

Personality
Diabel is a strong and natural leader, having led the raiding group with tactics, resulting with almost no casualties and inspiring fellow players to clear the death game with his speech of determination. However, his personality has a darker side as he secretly tries to use the situation to his own advantage, like attempting to purchase the Anneal Blade from Kirito to reduce his attack power during the boss raid and ensuring that he was assigned to a support role of handling Illfang's minions in order to prevent him from taking the Last Attack (LA) on the boss and then aiming for the LA himself.

Aria in the Starless Night
Diabel was a beta tester for SAO. Two months after the start of the SAO incident, Diabel became the leader of the raiding group that challenged the First Floor boss of SAO.

Secretly desiring prominence and identifying Kirito as his biggest rival, Diabel takes steps to hinder Kirito from getting the last attack on the boss monster of the 1st Floor; first by trying (and failing) to buy Kirito's «Anneal Blade» via Kibaou, and finally by relegating Kirito to the support role of fighting «Ruin Kobold Sentinels», the boss monster's guards.

During the boss fight, Diabel showed great skill in commanding the raiding group against «Illfang the Kobold Lord», gradually wearing down its HP with no losses. But as the boss's HP dropped to critical, Diabel charges forward in a bid to land the last attack himself, expecting the boss to switch weapon to a Talwar, which was the case during the beta. The boss instead produces a Nodachi along with a completely different attack pattern. Surprised and too late to react to the change, Diabel is struck down by a devastating combo from the boss.

As his HP bar vanished, Diabel asked Kirito to defeat the boss monster and save the others before dying.

Trivia

 * Argo identified that his name meant Devil in an Italian dialect (Romansch:Diavel).
 * Diabeł can also mean Devil in Polish.
 * The anime altered much of his background, by omitting the points about him trying to buy Kirito's sword to weaken him, and even making sure that Kirito would be assigned the task of handling the bosses minions to ensure that he wouldn't have the chance of dealing the last attack on the boss. Instead, the anime portrays Diabel as a selfless knight, though it did show him trying to get the Last Attack on Illfang.
 * Diabel proclaims himself to be a knight, even though there are no classes in Sword Art Online.