Answer: Klytaimestra
Step-by-step explanation:
In literature, a foil character is a the one that contrasts another one (the protagonost in most of the cases) and usually highlights his/her qualities.
In this case the character os Klytaimestra is a foil character of Penelope because the first one is an adulteress woman who not only cheated on her husband but also killed him. Her behaviour highlights Penelope's virtue and fidelity.