There will be more social shake-ups at Seattle Grace on Grey’s Anatomy.

Australian actress Melissa George guest stars starting with Thursday’s episode as Sadie, Meredith’s friend from the past. “I show up at her doorstep,” George tells PEOPLE. “[Sadie] gets a job at Seattle Grace due to her father, which is yet to be explored. She’s taking a leadership role at the intern society, which is hysterically funny. I haven’t stopped laughing for six weeks.”

Apart from inciting jealousy in Meredith’s current best friend Cristina Yang, Sadie’s character, who is bisexual, will be full of surprises?including possible romances, says George, 32. “I don’t know whether it’s male or female. It could be either; it could be both. We’re all dying to find out.”