Anna Ripley

Dr. Anna Ripley was a follower of the Briarwoods. After the Briarwoods seized control of Whitestone, Percy's backstory includes his early attempt to kill Dr. Ripley in Stillben, but he was captured by her guards, tortured, and interrogated. Dr. Ripley began gathering intelligence about Percy's weapons and experimenting with firearm construction, losing a hand during one of her experiments. As an NPC, Dr. Ripley is played by Matthew Mercer.

Fascination with Percy
Ripley has a fascination with Percy's construction of his own pepperbox. While investigating her room at Whitestone Castle, Percy found many sketches of it drawn from eye witness accounts.

She designed her own firearm, similar to Percy's pepperbox, but more refined and less styled. It has only one barrel and a small, rotating, four-cylinder chamber.

Work for the Briarwoods
She was working for the Briarwoods to create an acidic compound that would dissolve the white stone that Whitestone is named for and distil it into pure residuum, a very powerful magical essence used in constructions like the Ziggurat.

When her work was completed she tried to leave, but the Briarwoods threw her in the dungeon. Vox Machina found her there and begrudgingly let her out of her cell. She escaped Whitestone Castle while Vox Machina fought Professor Anders.

After Whitestone
Percy learned that Victor, the black powder merchant in Vasselheim, did business with a nicely dressed lady, most likely Ripley, several weeks before he returned.

Trivia

 * Dr. Ripley is the first character featured in Critical Role missing a hand, followed by Cerkonos, Tyriok Gadsworth, and Shale.  (Victor is missing two fingers from a hand, but he still has most of it.)