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I don't think I've ever done that. I am generally consistent in my likes and dislikes. On initially encountering a character, I either like them or try to suspend judgement. Whatever it is, is what it is. Sometimes I've disliked a character and then come to like them - that happened with Angel on Buffy, with Owen on Torchwood - but that's rare.
Moreover, I simply don't watch shows if they have characters I hate. If there's something - anything - I hate about a show, I stop watching it.
There are characters whom I've loved, who turned into twits. I always blame the writers, and it's usually the sign of a show in decay. Examples:
1. Fox Mulder in X-Files. He found out what happened to his sister and he didn't care. That was only one atrocity in the writing of X-Files at the time, but it was the last straw. (Do not mention the death of Krycek to me. Just don't.)
2. Michael Garibaldi in Babylon 5. He went from being one of the coolest guys on television to being Dagwood.
3. Wash on Firefly. His senseless death turned me off the character.
4. Amanda on Highlander, who was cool with Macleod but annoying on her own show.
There is a fifth example of something entirely different, and I may have trouble articulating this one. The character is the Tenth Doctor on Doctor Who. Yes, the one played so beautifully by David Tennant. You know I love him, right? He's adorable, he's smart, funny, tragic, mercurial and generally utterly delightful.
But there's a 'but'. Ten has treated characters I love badly - and it's to their credit that they handled it well and carried on with their lives and their ambitions. I'm thinking of Jack and Martha, here, primarily. Also Rose. I love his "I'm the Doctor and I'm ruthless and cruel" scenes, but his real cruelties happen when he's not paying attention, and when he's at a loss about how to handle a situation emotionally. Or when he fails to find a good solution to a problem. I'm thinking of Donna, there. And River.
So... it isn't that I don't love Ten, I do. I love Ten far, far more than I probably should.
It's that I don't always like him.