Mrs. Cheveley is the dark figure. She comes onto the scene with a self-absorbed mission: to establish her own wealth and security by unseating Sir Robert's. She's a shrewd woman who's probably done her research – unsettling Lady Chiltern is an extra treat. Her challenge to Sir Robert pits the Chilterns against each other.
The Chilterns' lived-in, happy marriage seemed to guarantee closeness, but Mrs. Cheveley's intervention has cracked open an area of deep misunderstanding between them. They feel like strangers to each other. At this point in the play, many negative outcomes are possible: Sir Robert may be exposed and ostracized, or abandoned by his wife; he may involve England in a fraudulent scheme. Mrs. Cheveley may get everything her corrupt heart desires.
Lord Goring is fighting Mrs. Cheveley, slowly revealing her true nature. She's the woman behind the drawing room door, and she's a trapped criminal: "her face distorted. Her mouth awry. A mask has fallen from her… " (3.284) He also reveals himself as "the philosopher that underlies the dandy" (4.236) in his advice to Lady Chiltern. As these characters assume their true identities, a reparation of everyone else's relations can occur.
The unwelcome interruption of Mrs. Cheveley has actually succeeded in helping the Chiltern marriage grow. Lady Chiltern sees her husband more realistically, and Sir Robert has cleared his conscience. Lord Goring and Mabel embark on a union that's likely to be a little unconventional.