I never expected her of all people to be a Kamitsuki…
Yep, Marie’s a Kamitsuki. Although even she wasn’t aware of that herself. It turns out that Pino ate them and used them as the Runes. During the attack, in the confusion, Saki gets stuck with A-drei.
A writing on the wall reminds A-drei of the time when their group had to kill one of them own for committing a capital offense.
Anyway, back to Marie: 5 years ago, she was the test pilot for the Valvrave prototype and Unit 1’s original pilot, where she and Pino became friends. The reason of Marie’s memory loss is because they’re being used for the Runes. The more she uses a Valvrave, the more Runes is consumed, and the more memories Marie loses.
Dorssia captured the students and Kyuma and Thunder pinned down, that only leaves Unit 1. Since Marie is inside, she has to pilot it. She does and unleashes more power compared to Haruto piloting Unit 1, at the cost of losing more memories at a faster rate.
After using an even stronger Harakiri Blade to wipe out the remainder of the Dorssian army, it ends up taking the rest of Marie’s memories… and her life.
Yeah, who’d expect that such a minor character had such a big role? This is like all of Sunrise’s standard writing in one go.