Well Geass really messes around with standard mecha character roles so its expected. As the main pilot for the protagonist's side Kallen more or less takes the spotlight whenever battle comes up. So it's not unexpected that she's very important. She's more or less the 2nd most important female in the show right now and I doubt that will change since Euphie was the only character she was competing for that spot with and she's gone now. I don't expect another new female character to take that position so more or less C.C. is the heroine and Kallen is the secondary one.
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