Mason Drake from Lois & Clark: she was the blonde district attorney who hated Superman but had a huge crush on Clark in season two. It was such a relief to have a recurring character who wasn't blind to the fact that Superman had no personality, but Clark was the hottest thing on wheels. I always wanted Clark to give her a serious chance, not only because I wanted Lois to be jealous, but simply because she was a great girl. Too bad she died and no one cared two episodes later.
Bussy, I'm so glad you mentioned Mayson Drake. She was such a cool character, and I was so disappointed that the only thing they could think of to do with her as a character was kill her off. But hey, at least it took two episodes for everyone to forget her; poor Alicia Baker was forgotten the episode after she died. And she was another character who had such potential and whom they just killed off.
Of course, I also have to mention a whole host of characters from Veronica Mars. I really liked Rebecca James, and I thought she made a much better love interest for Keith than Alicia did. I know there was the whole problem that the actress got pregnant, but I never understood why they couldn't have brought her back in season two. Of course, I fully expected that Veronica's mother would play a major role in season two, but apparently they decided that throwing a bunch of extras off a cliff and spending half a season on a pregnant girl in a coma would make a better story.