Fiona Frost aka Nightfall is a member of WISE. She was trained by Twilight to be an spy and she fell in love with him, but the feeling was not mutual, even if Twilight respecte her as an spy.
When Twilight had to pose as Doctor Loid Forger and use a fake family to infiltrate the circle of an important politic Nightfall was not at hand to pose as his wife. She resents Yor Forger who is the stand in wife of Loid and want to replace her, so she can act as Loid wife and express her real feelings.
She considers Yor Forger as inadequate as Twilight wife and had an obssesive intention of replace her. After she proposes a “friendly” tennis match, she was overcomed by Yor superior strengh. With her broken hart, Fiona conceded victory and quickly retired to cry her heart out. Defeated in battle but not in war.
Only Anya Forger knows the real feelings of Fiona for Loid.