The "Game of Thrones" alum will play Maia, Diane Lockhart's (Christine Baranski) goddaughter who joins her firm just after passing the bar, and is a key part of Diane's plan for the future, reported Aceshowbiz.
The spin-off also stars Cush Jumbo, who will reprise her role as Lucca Quinn, and Sarah Steele, who will return as Eli Gold's (Alan Cumming) smart and outspoken daughter Marissa Gold.
"We're thrilled to have Rose on board. We needed a young actress who could hold her own with Christine Baranski and Cush Jumbo, and Rose is perfect. She's real, she's strong, and she can play comedy. If we designed an actress from the ground up, we couldn't have done better," executive producers Robert and Michelle King said in a statement.