Oh, you mean like how dad's girlfriend is always smiling and says nice things, but deep down, you just know she's a bitch?
Brandi was Karl Mayer's mistress.
Brandi is Karl's secretary and mistress. As a result of Karl's affair with Brandi, Susan and Karl divorced a year prior to the start of the series.
In "Pretty Little Picture", Brandi apologizes to Susan for having ruined cheated with Karl, explaining that she thought their marriage was already over.
Later in the first season, in Move On, Karl reveals that Brandi has left him sending him back briefly into the arms of his wife. When Karl asks Susan for a second chance, she declines, stating that she is committed to Mike . The character is referenced several times throughout the series, usually when Susan and Karl get into childish arguments.