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.

Alex Danvers is the only biological child of scientists Eliza and Jeremiah Danvers. When Alex was a teenager, her parents adopted an orphaned Kara Zor-El and they grew up together in the town of Midvale. Although Alex was an exceptional student, particularly in science, she couldn't help but envy her Kryptonian sister's alien abilities. Alex was relieved when Kara decided not to use her powers, but still felt embarrassed by her alien behavior. Nonetheless, as the older sister figure, Alex felt protective of Kara and sympathy for her losses and displacement.

At some point, Kara took a reluctant Alex flying through the night and they were spotted by the Department of Extranormal Operations (DEO), a secret government agency tasked with monitoring alien presence on Earth. Unbeknownst to the young girls, Jeremiah agreed to work for the organization under Hank Henshaw instead of letting them take Kara away.

In high school Alex had a best friend, Vicky Donahue. She enjoyed sleepovers with Vicky and developed feelings for her. Alex recognized her feelings; however, she got scared and was suddenly fighting with Vicky. This falling out was the reason why they never talked again.

Suffering the loss of her father and her suppressed homosexuality, among other factors, caused Alex to excessively drink and party as a young adult. Nonetheless, she managed to earn dual doctoral degrees (Ph.D. and M.D.) before being recruited into the DEO herself by Henshaw (later revealed to be J'onn J'onzz (who promised Jeremiah that he'd look after his daughters). In season three it is revealed that, at some point in the past, she was a practicing physician and worked at a hospital in Seattle.

At the start of the series, Kara is initially unaware of her sister's true occupation. Alex reveals herself to Kara as well as the existence of the DEO after Kara saves Alex from a near plane crash and decides to become a superhero like her cousin, Clark Kent / Superman. She is also Hank Henshaw's trusted right-hand operative. Alex is tasked with training Kara in combat and provides her with a Fortress of Solitude-esque room with a projection of Kara's mother, Alura. Alex, J'onn, and the rest of the DEO work alongside Kara, dubbed "Supergirl" by the press, to put away dangerous alien criminals who have escaped from the Fort Rozz prison spaceship, including Alura's identical twin and Kara's aunt, Astra. Alex puts on a suit with kryptonite lacing and fatally stabs Astra with a kryptonite sword. Jonn took the blame for Astra's death, because he didn't want Alex to lose her sister. However, Alex felt guilty and although she knew she could lose her sister with her confession, she told Kara, against Jonn's will, that she had killed Astra in self-defense, and not Jonn. She explained that she had no choice since Astra wanted to kill Jonn and apologized, crying bitterly to her. At first it looked like Kara was running away and couldn't forgive Alex, but she finally hugged and comforted her, showing that she had accepted her sister's apology.

While continuing her work at the DEO, Alex crosses paths with National City Science Police officer Detective Maggie Sawyer. The two initially come into conflict over work, but quickly form a flirtatious friendship and Alex begins to develop feelings for her. That forces Alex to again confront her homosexuality, and her true feelings that she suppressed since her youth. Again Alex denies that she is a lesbian, but after a long conversation with Maggie, she finally admits it to herself for the first time. Maggie therefore encourages her to stand by her sexual orientation and to tell her family. After a few days Alex finally gathers up all her courage and manages to come out to Kara. Her sister is at first a bit shocked about it, but has absolutely no problem with this and has been supporting her ever since, just like her mother and friends who were told about it after Kara. So Alex comes out as gay at the age of 28 and enters into a relationship with Maggie. During the episode "Alex", Kara and Maggie work together to locate a kidnapped Alex. At the episode's conclusion, Kara destroys the tank designed to drown Alex. Alex and Maggie profess their love for each other, and they become engaged in the season finale. Alex also learns of her father Jeremiah's mysterious survival and ambiguous partnership with the anti-alien organization Project Cadmus.

As Alex and Maggie prepare to get married, Alex discovers that Maggie does not ever want kids. Alex, realizing that she does want kids someday, breaks up with Maggie and they reluctantly go their separate ways. In the aftermath of the breakup, Kara invites Alex to her friend Barry Allen's wedding on Earth-1, where she ends up having a one-night stand with Legends of Tomorrow's Sara Lance (Caity Lotz), both are soon forced to fight off an invasion of Nazis from a parallel Earth together. After the defeat of Reign, J'onn steps down and promotes Alex to Director of the DEO.