The problem with that theory about Lyon is that there are a few scenes of him where he's not around anyone else (except maybe Riev and Morva I guess) but in Eirika's route he's still doing the whole "rar i eat humanity" routine and on Ephraim's route he's still doing the "I'm going to save my country even if I get destroyed and I hope Ephraim stops me if I get consumed" thing.
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We clasped our hands, our hands in praise of a conquerors right to tyranny