Chester is the main antagonist of the last episode of the Proud Family called "Psycho Duck" (not counting the film, which served as the true series finale). He was a duck who Penny Proud rescued from the pond, but when she took him home due to finding out he couldn't swim or do anything a normal duck would do (accoding to Trudy, who is a veterenarian and is the one who pointed it out since Penny took him to the vet), he started torturing Oscar behind the backs of the rest of the family whom he acted out as a saint to them, and later on did the same to them to the point he took over the house.
Why He Sucks
He is a reckless, noisy, loud, spoiled, and evil duck who tried to make Oscar's life miserable by eating his food, attacking him behind the backs of his entire family, and pretending to be good to Penny, Suga Mama, Trudy, and the twins. To add salt to it, nobody (including Trudy, who trusted and pittied Chester the most) would let Oscar get rid of him and didn't believe him about Chester and though he was just being crazy.
He not only stood up to the Gross Sisters and protected Penny and her friends when they were on their way to school, he made the stupidest choice to attack Penny and her friends when they did "absolutely nothing to him" to make him like that to them, resulting to the gang being scared of him.
He later on took over the house and bossed everyone around, to the point it revealed that Oscar was right about his true nature (hence, they wanted to get rid of him).
He managed to escape from Canada back to the US so he can torment Oscar and his family again.
He is also rude, obnoxious, egotistic, greedy, and insensivtive.
The Only Redeeming Qualities
He finally got his comeuppance when Bobby Proud helped the Prouds get rid of him by disguising as a female duck to make him fall in love with "her", and when he tried to kiss Bobby, he got into a mishmash when he found out it wasn't a real duck by fighting Bobby, but he looses him and drifts off in the lake never to bother the Prouds ever again.
In the end, he was revealed to be an Easter Duck who belonged to a rich man and was returned to him by Wizard Kelly (implying he must've found Chester somewhere and gave him to his rightful owner), and since he was an Easter Duck, it meant he was raised by someone when he hatched and never got the chance to learn or be familiar to his duck instincts, explaining his backstory.