Mei Wu (Susan Park) is the main villainess from "Hong Kong Hustle," episode 7.17 of Castle (airdate March 16, 2015). She is one of the waitresses working at the Jade Temple Restaurant, which is owned by Mimi Tan.
As the episode revealed, Mei Wu was one of many Chinese immigrants who were brought into the States by Mimi via a human trafficking operation and had to work off their debt to her. Among the others was Mei's childhood friend and fellow waitress Zhu Yin. However, Mei Wu noticed that Zhu Yin was falling in love with Henry Morgan, a frequent customer who attempted to buy her freedom from Mimi, who ordered him to plant drugs on her business rival, Jeffrey Wadlow, in exchange. After Henry did so, he arranged for Zhu Yin to meet him at a nearby park late one night with the good news, which angered Mei Wu, as she felt that Zhu Yin was all she had and feared being alone without her.
After overhearing their conversation, Mei Wu falsely told Zhu Yin that Henry left a phone message cancelling their plans to meet. Mei Wu then turned heel by shooting Henry to death at the park after a verbal confrontation. Henry's death brought Chief Inspector Zhang from Hong Kong to New York, and she was shown searching his body before holding a jogger at gunpoint, making her a suspect until she was identified as a cop. While working the case with Richard Castle and Kate Beckett, Mimi was suspected in the murder after her own villainous reveal, as well as Zhu Yin's confrontation with her.
It was after Zhu Yin's statement about the phone message that Mei Wu was revealed as the true killer, and she initially continued her lie when she was brought in for interrogation. Castle and Beckett exposed her deception and revealed evidence of her actions--blood on Mei Wu's clothes that was a match for Henry. The villainess remained defiant, but after Zhang scolded her in Mandarin, the evil Mei Wu confessed, justifying that Zhu Yin was all she had in the States and she had to do something to prevent her friend from leaving her. Following her confession, Zhang handcuffed and arrested Mei Wu for Henry's murder.