I would have to say Sissi, because she's just a spoiled rotten brat; those kind of people annoy me.
"We are the Borg. Lower your shields and surrender your ships. Your biological and technological distinctiveness will be added to our own. Your culture will adapt to service us. RESISTANCE IS FUTILE." - Borg Collective
I also agree that Sissi is overrated, but she has a sweet side to her (though rarely seen). Jim, too, needs better development as well. With Jeremie having a severe distrust in the big guy, and Jim being what CL.com calls a "Moronic Musclebrain", well...case in point.
I voted for William. Why? Because he played the "bad boy/tough guy" act way too strongly early on, and what happens when he goes to Lyoko? He owns everyone and everything. First the monsters, then the Lyoko crew when he got possessed by Xana, and finally Lyoko itself. I realize it was the plot and all, but come on. All this time, Xana keeps failing in virtually all of his plans, and then when he takes control of a newbie to Lyoko, he completely demolishes all the experienced warriors?
Yeah. Ulrich is kind of overpowered (which sort of makes sense, since he really only has melee attacks), but it's like William was using god mode when he first got to Lyoko.
XANA Mode, you might say. And to be fair, he DID lose to Skippy.
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"There are advantages in being honest about your own limitations: you gain the right to talk about the limitations of others." - Science of The Discworld 3