The Archangel Auriel is a member of the Angiris Council. She is the Archangel of Hope.
Auriel in the Book of Cain
It is through the archangel Auriel that the power of hope flows into the fabric of creation. Her eternal light illuminates even the darkest souls. With Malthael's departure, it falls to Auriel to hold the Angiris Council together. Should her light ever fade, all Heaven would fall to despair. - Diablo III
History
Auriel is the only female member of the Angiris Council. She is described as being dressed in soft blue robes, and due to her sympathetic nature, contrasts sharply in domain and character with the fierce and arrogant Imperius. It has been observed that Itherael is closest to Auriel among the Archangels, possibly because Hope is the only comfort to uncertain Fate.
Auriel presides over the Gardens of Hope, a place of healing and spiritual reflection in the High Heavens. In combat, she wields Al'maiesh, the Cord of Hope, which whips through foes and burns them with righteous fire. Al'maiesh can also be used to grant clarity of thought and emotion.
When Sanctuary was discovered by the High Heavens, she was first and the most passionate of the Angels who defended the existence of the hidden world. She sought to uphold the inhabitants' right to life and wanted to see them grow and mature.
Relationship with Mephisto
It is speculated that Mephisto and Auriel may have once been lovers.
"We, like you, made false assumptions about this place, false assumptions about the creatures spawned by both our kind.' A blazing red eye materialized, then winked in the direction of Auriel, who utterly ignored it." -Mephisto, pg 332 of The Veiled Prophet
Also, when the Kiss of Mephisto (used to resurrect the Morlu), it is said that it was named 'Kiss of Mephisto' because it was originally a gift for someone else, other than Lucion.
"Turning his eyes back to the cataclysmic sight below, Lord Lucion gestured toward the very center, where the worst of the carnage had and was still taking place. In the midst of everything, a black gemstone nearly as large as a man sat embedded in a triangular column of red-streaked marble. “Blood marble,” it was named, naturally. The stone was called by Malic’s master the Kiss of Mephisto, although the cleric had, from past comments, reason to believe that it had once been named for another of whom Lord Lucion would not speak." -Birthright
Taking Mephisto's flirtatious attitude towards Auriel into account, some have suggested that the Kiss of Mephisto might have been intended for Auriel.
On the other hand, there is a somewhat simpler explanation. Mephisto's lecherous behavior could have just been a barb to her status as the only female on the Angiris Council.
Diablo III
Auriel in the Opening Cinematic
Auriel, along with Tyrael, Imperius and Diablo himself, has been confirmed to appear in the third game.
In the Opening Cinematic, she is one of the Angels seen to clash with the endless hordes of the Burning Hells, apparently using her Cord of Hope to inflict massive damage on them.
In Diablo III: Wrath, Auriel is the first Archangel of the Angiris Council to assault the forces of Hell with the aid of Al'maiesh, and later helps save Imperius by using the Cord of Hope to break Diablo's hold upon him. As the Council overpowers Diablo, Auriel binds him with chains and notes that, "At long last, we have captured one of the Prime Evils. Holding him will surely turn the tide of war in our favor." However, a brash Imperius, ignoring this and Tyrael's objections, brutally slays the constrained Lord of Terror, a decision that Auriel denounces as "sacrilege".