Diavel is a character from the anime Sword Art Online, a player who became trapped in the deadly virtual reality MMORPG of the same name. As one of the few participants in the game's closed beta test, he possessed valuable knowledge that most other players lacked when the death game began. Physically, Diavel is recognized by his bright blue hair and matching blue eyes, and he typically wears a set of bronze armor over a blue undershirt, carrying a longsword and a kite shield.

In the early weeks of the Sword Art Online incident, as fear and distrust among the ten thousand trapped players ran high, Diavel emerged as a natural and charismatic leader. He took the initiative to organize the first major raid against Illfang the Kobold Lord, the guardian of the first floor of Aincrad. His leadership was marked by careful planning and tactical coordination, which helped the raid party wear down the boss's health with almost no casualties. He made inspiring speeches to rally the players, presenting himself as a selfless knight determined to clear the game and free everyone. This public persona made him an effective unifying figure during a critical moment.

However, beneath this heroic exterior, Diavel had a more calculating and self-interested side. Secretly, he aimed to claim the Last Attack bonus for defeating the floor boss, a reward that would bring him significant prestige. He saw Kirito, another skilled beta tester, as his primary rival. To reduce the chances of Kirito landing the final blow, Diavel attempted to purchase a powerful sword, the Anneal Blade, from Kirito through an intermediary, Kibaou, in order to weaken him. When this plan failed, Diavel assigned Kirito and another player, Asuna, to a support role during the boss fight, tasking them with handling the boss's minions rather than engaging the main target. These actions were a calculated effort to ensure he would be the one to secure the glory of victory.

The boss battle against Illfang proceeded smoothly under Diavel's command until the kobold lord's health dropped to a critical level. Based on his experience in the beta test, Diavel expected the boss to switch its weapon to a curved talwar. Confident in this knowledge and seeing his chance, he charged forward to land the killing blow. To his shock and fatal surprise, Illfang instead produced a nodachi, an entirely different weapon with a new, unfamiliar attack pattern. Unable to react in time, Diavel was struck down by a devastating combo. As his health bar depleted, he revealed to Kirito that he was also a beta tester and entrusted him with the task of finishing the boss and protecting the other players, asking simply that Kirito save them. His final moments shifted from self-interest to a sincere plea for the group's survival.

Diavel's death had a profound and lasting impact on the player community. The raid party was shocked, and his demise reignited widespread hostility and suspicion toward all beta testers, whom some players blamed for holding back crucial information. In response, Kirito publicly declared himself a beater, a derogatory term combining beta tester and cheater, to absorb the community's anger and shield other former testers from persecution. Furthermore, Diavel's death fractured the fledgling clearing group. His legacy inspired two of his followers, Lind and Kibaou, to form competing guilds: Lind established the Dragon Knights Brigade, while Kibaou founded the Aincrad Liberation Squad. These two guilds would become major forces in the subsequent efforts to clear the game's floors, though their rivalry often complicated the task. As for abilities, Diavel was a capable swordsman and a skilled strategist. His experience as a beta tester gave him deep knowledge of enemy patterns and dungeon layouts, which he used to lead the raid effectively until his fatal miscalculation. His coordination and command skills kept the chaotic group of players organized against a formidable enemy, proving his competence even as his personal ambition ultimately led to his downfall.