The Twi'lek bounty hunter in “The Prisoner” is Xi'an.
Xi'an appears in Chapter 6 of The Mandalorian, released on Disney+ on December 13, 2019. She is a purple-skinned Twi'lek mercenary and a member of Ranzar Malk’s crew, which is hired to rescue her brother Qin from a New Republic prison transport.
Xi'an has a past relationship with Din Djarin, giving the mission a personal edge. She is skilled with knives and repeatedly tries to turn the crew against Din. When the group betrays him, Din survives, defeats the mercenaries, and leaves Xi'an locked aboard the transport for New Republic authorities to find.
A common mix-up is confusing Xi'an with Hera Syndulla, another prominent Twi'lek in Star Wars. Hera is a Rebel leader and pilot from Star Wars Rebels, while Xi'an is a criminal mercenary introduced specifically in The Mandalorian. Xi'an is played by Natalia Tena, known also for Game of Thrones and the Harry Potter films.