I know a bunch of characters that have gotten worse later in certain series:
Daffy Duck(Looney Tunes) - Daffy was wacky/zany/crazy way back in the old school shorts, but now he's become a greedy/selfish jerk along with his rivalry with Bugs Bunny.
Lois/Brian(Family Guy) - She was a sweet/caring mother, but now she became terrible and a bigger jerk than Peter on bashing Meg, Brian, and poor Joyce.Brian was great/the voice of reason, but he went downhill on being an annoying liberal and having bad luck with women, but his moments with Stewie were the best though.
Bloo(Foster's Home For Imaginary Friends) - He was great in the pilot movie, but he became a humongous jerk for poor Mac and his friends throughout the series which made me stop watching the show.
Candace(Phineas And Ferb) - Agreeing with RonDrakenfan17, she may be hot/funny at season 1, but further on seasons 2/3, she became more annoying/crazy on always wanting to bust poor Phineas & Ferb especially when her brothers help her out, she still wants to bust them.
Misty/Team Rocket:Jessie,James,& Meowth/Ash's Charizard/Max/Iris(Pokemon/Black & White) - Both Misty/Ash's Charizard/Max were so annoying/terrible to poor Ash, but at least they became soft at the end though.The three losers were funny comic relief villains way back, but further on they became unfunny/dull/boring now and didn't care about their serious personalities.Iris may become worse further on being annoying/smug/obnoxious, always using her annoying "you're such a kid" crap, but I still like her despite her lousy attitude and has her funny/great moments at times.
Tyson(Beyblade:V. Force/G. Revolution) - He was awesome/likable for caring for his friends in season 1, but further on seasons 2/3, he became so unlikable/awful on being a gigantic butthead towards his friends at times along with his major hard-headed stubbornness as much as I feel sorry for poor Kenny/Max/Ray/Kai to put up with him.Especially with Max/Ray leaving him, he was still terrible which makes me wanna punch him in the face, but I'm glad Ray/Lee kicked his butt at the end since Ray was so awesome.
Sasuke/Sakura(Naruto/Shippuden) - He was great in the beginning, but further on becoming a vengeance-loving jerk and not caring about your comrades totally made him went downhill.I used to like her, but not anymore when she became the biggest jerk ever in the original series along with making her useless, but in the next series with her lying to poor Naruto on loving him was really hurtful/not cool, but I pity her because of Sasuke becoming a vengeance-loving criminal.
Kiyomi/Ray/Naomi/Misa/Namikawa/Aiber and Wedy/Matt(Death Note) - Don't know if they count but Kiyomi, Misa, and Mikami were awesome, but sad that they're Light's servants with no respect to the end.Ray/Naomi/Namikawa/Aiber and Wedy/Matt were such awesome minor characters until they became Light's victims to be killed off which is so horrible/not cool, I'm so happy Light got his butt kicked at the end.
Madarao/Tokine(Kekkaishi) - They were pretty cool at the beginning, but they became such awful/unlikable jerks being a pain for poor Yoshimori, but yet I still like Tokine because of her great skills and funny fear of bugs like Misty, hahahahaha.
Mepple(Pretty Cure/Max Heart) - He was pretty cute/funny at the start of the series, but further on he became so terrible/unlikable on being a huge jerk for poor Natalie.Natalie putting him in his place was awesome, but Mepple doesn't know when to quit on always bashing on her which he irritates me


!!!!!For some reason Mepple so reminds me and acts like Bloo on the whole "making your owners' life miserable" formula.