Kelley Mack. (Barry Brecheisen/Getty Images)Kelley Mack, an actress best known for her role as Addy on the television series The Walking Dead, died last weekend, her family announced on Tuesday. She was 33.Mack was diagnosed with glioma, an aggressive tumor that affects the central nervous system, last year."It is with indelible sadness that we are announcing the passing of our dear Kelley," her sister, Kathryn Klebenow, said in a statement posted to her Instagram page. "Such a bright, fervent light has transitioned to the beyond, where we all eventually must go.