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Kozutan: It's the emperor himself, likely taking the idea of her dying her hair as disguise to move incognito.
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Angelsun: The way I understand it, the emperor was nearby and might've even been with her part way, and he let her into the academy alone to gather her things while he wait outside.
Then when he sensed magic being used, he disguised himself and came to check on things.
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karolyn: Considering he plan to make her his (fake) fiancee, this would be a good starting point to see what kind of punishment he have in mind for nobles aiming for his lover (the magician already got his magic removed/broken)