Pretty and Eugly are fraternal twins with opposite personalities. Pretty is usually not very nice to Eugly, but Eugly is very nice to Pretty.
Pretty treats Eugly like a slave or bodyguard more than a sister. She forces her to obey all her orders and cruelly insults her almost all the time. However, Pretty does not like it when anybody else tries to insult Eugly. A notable example of this coming into play is in one episode where Stumpy insults Eugly and Pretty punches him, but immediately turns to Eugly and tells her Stumpy was right. In the episode "Let's Play Marriage", Pretty forces Kaeloo to allow her to marry Quack Quack despite not actually liking him. When Eugly, who genuinely was in love with Quack Quack, started to cry, Pretty hypocritically accused her of being an egoist.
Eugly usually follows Pretty's orders without hesitation. If anyone tries to hurt Pretty, Eugly usually beats that person up. A rare example of Eugly defying Pretty's orders was seen in "Let's Play Castaways". In this episode, Mr. Cat and Quack Quack were stranded on an island, and Eugly suggested rescuing them. Pretty refused, since she considered this to be a good thing, but Eugly grabbed her and forced her to come along on a rescue mission. Interestingly enough, at the end of the episode, Eugly leaves Pretty on the island and forgets to rescue her (and Mr. Cat) until reaching home. In "Let's Play Orchestra Conductor", Pretty gets angry at Eugly for not choosing her to be the "orchestra conductor" and acts as thogh she betrayed her.