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Misty had a defined personality and managed to contribute a lot of entertaining moments to the show in the early episodes. Her constantly arguing with Ash or needling Brock was always pretty funny and very rarely annoying- Misty having an antagonistic side yet being unwilling to see how much of a pain in the ass she is gave her dimension. There was also a lot of depth to her in that she had something to prove to her sisters and herself. Okay, sometimes she was annoying as Rachael Lillis' performance was quite often very shouty but I don't think that's necessarily a bad thing- she was, after all, a very shouty character with a temper as fiery as her hair. She mellowed later on and matured, something we can actually thank Togepi for.
However, Togepi marked the beginning of the end for Misty. She went from being a Trainer to being a babysitter. For a while this even rubbed off on Pikachu and while some of you may not agree that Pikachu is important as an actual character, Pikachu still has a defined personality and to see him reduced to shouting "PiPiPi!" whenever Togepi wandered off was also detrimental.
Togepi could have been a five-to-ten episode arc and created just enough mystery for Gen II and eggs. What we got lasted nearly five YEARS. Check out the Whirl Islands mini-arc, look at how good Misty is in those episodes. She hadn't been that entertaining in seasons at a time, she had no defined purpose.
May was a breath of fresh air because she had a defined purpose and direction, something Misty had lacked almost since the get-go. Misty in Chronicles, however, doing Gym Leader shit was pretty fun. Just goes to show how good a character can be when you give them something more to do than stand at the sideline holding an egg.