Elistaria

Elistaria is the dark lady of Windemere, and a very prominent member of the Oppression.

The Avatar sought her out in Ultima V, to acquire the Black Badge. Upon receiving the Oppression's password, Elistaria gave the badge to the Avatar, officially welcoming the hero into the organization. A self-proclaimed practicioner of the darker aspects of magic, she also spoke of her fondness for conjuring Daemons and the like. Her "best two summonings" were seen guarding the keep, poised to destroy any intruders at her command.

She told the Avatar to ask Thrud for some special items (the Jeweled Sword and Shield) that might prove useful in the hero's "infiltration" of the Resistance.

In Lazarus
Elistaria has been more fleshed out in the Lazarus version of the game. Here she's actively training dark mages for the Oppression, revealing herself to be a cruel and dominating woman. Her background is also much expanded-upon.