Sage is the brainchild of Los Angeles–based actress, writer, breast cancer survivor and flat advocate Renee Ridgeley, who also voices the character, and who says that while Sage's role is small, it's "crucial," as she's the person Marge and Lisa meet with in order to work out some issues relating to body positivity. Ridgeley, who has many connections in the entertainment industry, has been "hounding different people," she says, "one of them being my husband," Matt Selman, who is The Simpsons showrunner. "I was like, 'Let's get a breast cancer survivor on The Simpsons!'"
...
So it was "kismet," the longtime show writer continued, when they started working on a new episode written by Juliet Kaufman, which had a theme of body acceptance. "This is, like, the perfect episode [for Sage]," he [Selman] says. "It's like a grace note."
...The episode, she [Ridgeley] tells Yahoo Life, is "very emotional."