A'dal
Not much is known about A'dal's history or personality, only that he is the leader of the Naaru, a race of naturally peaceful beings made of pure light energy.
For a short time period, the ruins of the city of Shattrath were under control of the demonic Burning Legion. A'dal and his allies assaulted the ruins and reclaimed the holy city, where he ruled over the peaceful Sha'tar, a group of Naaru who came to free the Outlands of its evil, demonic inhabitants.
Not long after the Second War, the mage Khadgar found Shattrath and met A'dal, who taught him more about the Holy Light than he already knew. A'dal ruled over the Outlands in peace, until the demon, Illidan Stormrage, arrived and drove Pit Lord Magtheridon out of the Black Temple, thus ruling as the self-proclaimed Lord of Outland.
Years later, the demon lord Kazzak reopened the Dark Portal, a gateway which led to the Outlands. The heroes of Azeroth followed the demon into the shattered realm, where they made war against the forces of Illidan's Illidari and Kazzak's Burning Legion. Eventually, the heroes met A'dal, who welcomed them into the sanctuary of Shattrath City.
Years prior, the Naaru space ship of Tempest Keep had been taken over by the Sunfury Blood Elves, led by Prince Kael'thas Sunstrider. A'dal sought to take Tempest Keep back, and when the heroes from Azeroth arrived he put together a set of tests to find those worthy of storming Kael'thas Sunstrider's domain. The tests required the heroes to storm free Kargath Bladefist's hostages, free Millhouse Manastorm from the Arcatraz, recover Kalithresh's trident and Murmur's essence, and slay the mighty Magtheridon. Upon completing these trials, A'dal crafted a key which the heroes could use to enter the Tempest Keep. The heroes then proceeded to fight Kael'thas in his domaine, and defeated the Blood Elf Prince and his lackeys. The heroes brought one of Kael'thas's magical orbs back and proof of his defeat, but A'dal was contacting by the Elf himself telling him that he survived. Kael'thas then told A'dal and his advisors that he defected from the Illidari and sided with Kil'jaeden, the master of the Burning Legion.
Years later during the War in the Frozen Wastes, a Paladin named Crusader Bridenbrad had been infected by the Plague of Undeath. The Highlord of the Argent Crusade, Tirion Fordring, attempted to heal him with the help of some of Azeroth's greatest powers of nature, including Keeper Remulos and Queen Alextrasza. When both of these failed, Tirion sought the aid of A'dal himself. A'dal arrived, knowing that not even he can heal Bridenbrad. Instead, he cleansed Bridenbrad's body, making him immune to undeath.
Today, A'dal works to form the Army of the Light, which is said to rise against Kil'jaeden's evil and bring an end to his demonic armies and destroy his master, the Dark Titan Sargeras.