The term “foil” is defined as “To prevent from being successful; thwart.” It means that someone is to stop someone from doing or achieving something. In Doll House, the foil to Torvald Helmer is his employee, Krogstad.
Torvald and Nora are about to enjoy another Christmas holiday and more wealth in Torvald’s promotion when Krogstad comes into their lives with hopes of getting Nora to convince Torvald to let him keep his job.