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Isobel Flemming (Mia Kirshner)
We loved her because: Isobel hooked up with John Gilbert when she was a teenager, and from there, she really had no hope. He not only knocked her up and spawned the Doppelganger, but he introduced her to the world of vampires, and she became a little too obsessed. She was basically the worst wife ever to Alaric, and the worst mom ever to Elena, but her extensive vampire research did come in handy. So there's that.
We lost her because: Once you get mixed up with Klaus, you're doomed. He compelled her to help him capture Elena and that freaking moonstone, and then when he was finished using her, he compelled her to kill herself, right in front of Elena. Carol Lockwood's next party should really be a fundraiser for the Send Elena Gilbert To Much-Needed Therapy Foundation.