She saw some of her self in Kara, Clark Kent’s hot mess of a cousin who drunkenly collects her dog Krypto in the final scene of “Superman,” and spends a good amount of “Supergirl” in red-sun-planet party mode. But her bluster masks a traumatized, resilient and, ultimately, kind center. “Very beautifully and very surprisingly, she was someone who was just so close to the bone and who I am fundamentally as a person,” Alcock said.

Born in Petersham, a formerly gritty, now gentrified suburb of Sydney, the actress grew up with two younger brothers, Eddy, who now works in agriculture and sustainability, and Bertie, a rugby player for the Western Force in Perth. Always more interested in acting than academics, Alcock quit high school in 2018 when she won the role of the brash teenager Meg on the Australian TV show “Upright.”

“I tried really hard” in school, “but my brain just couldn’t do it,” said the actress, who says she has A.D.H.D. “Sometimes in the educational system, if you don’t do well at school, you’re seen as a naughty kid. But I just really wanted to be good.”

Her mother, Emma, a former nanny who got a Wonder Woman tattoo long before her daughter was in the DC Universe, encouraged the leap. “She’s a legend,” Alcock said, before lifting her hair and twisting around to expose an “E” tattoo on her upper back. “Because she’s got my back,” she explained of the ink. When asked how her mom feels about her success, Alcock beamed. “She’s just so proud.” (And yes, Emma is planning on making Supergirl her next tat.) The actress is more reticent when it comes to her father: “It’s not my story to tell.” She said at another point, “We love family, but also, families are hard, you know?”