And Celes definitely didn't start out because she wanted revenge immediately on Kefka. She was about to kill herself because she thought there was no hope left. She went out because she had hope that Locke was still alive, and she wanted to find him. They all want to stop Kefka, but I feel like she cared more about finding Locke than stopping Kefka, and I don't think she wants to stop Kefka because he's Kefka and she hates him. She wants to stop him to save the world, like everybody else.
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http://i.imgur.com/CbfFb.jpg