Ella

When the party owned Troll Skull Tavern in Ciniath, they hired several people to work at the tavern, and Ella was one of them. Unbeknownst to the party, she was a member of Jarlaxle's guild and a plant. When they discovered she was not who she said she was (after she had worked there for a while), they fired her.

She is with Jarlaxle Baenre at his table at the Wheldon Ball in Episode 19, disguised as another noble. Presumably she is some kind of caster, as she is the one who noticed that Durnan had a hole in his memory.