JULIET RYLANCE AS DELLA STREETPerry’s headstrong accomplice in solving crime, a legal secretary for aging attorney E.B. Jonathan (John Lithgow), she embodies modernity in her smart, well-tailored attire.Potter said: “Della’s backstory was that she came from a family that had money, but she made the decision to step away from the family and go out on her own to start work and pursue other things that interested her.