The Walking Dead writer Scott M. Gimple is paying tribute to Kelley Mack after the actress' death at 33 years old. "Sad news about a massively talented member of the TWD family," Gimple, 54, wrote via X on Wednesday, August 6. "We're grateful we got to work with Kelley Mack and share her magic with the world." Mack was best known for her role as Addy in season 9 of the post-apocalyptic horror drama. Mack's sister, Kathryn, confirmed her death via a post on the 9-1-1 alum's Instagram page on Tuesday, August 5.