Alexandra "Alex" Danvers Ph.D. M.D., also known as Sentinel, is a fictional character from the Arrowverse television series Supergirl, portrayed by actress Chyler Leigh. Created by executive producers Greg Berlanti, Ali Adler, and Andrew Kreisberg, the character was introduced in the pilot episode as the elder adoptive sister of protagonist Kara Danvers / Supergirl. Alex is a bioengineer, physician, scientist and a field agent of the DEO, a secret government agency that works to protect the Earth from extraterrestrial threats. She later becomes the director of the organization at the end of season three, after having been J'onn's second-in-command for years, and remains so through season five. Alex is also featured in the show's digital tie-in comic book series, Adventures of Supergirl, as well as the 2017 four-part Arrowverse crossover event "Crisis on Earth-X", with Leigh reprising her role across The Flash, Arrow, and Legends of Tomorrow.