Kokoro discovered baby-making and this was something Nana and Hachi weren't going to allow.
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Nana suddenly had a flashback to a traumatic memory as a Franxx pilot, which was a sign that she had "relapsed" into puberty. There's lots of hints here about how the current state of humanity is genderless, sexless, and reliant on asexual reproduction.
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In the end, Hiro's idiosyncratic group rediscovered the reproductive instinct through Kokoro and Mitsuru. Futoshi was always going to be cuckolded from the very beginning.
😱
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