https://www.dlsite.com/maniax/work/=/product_id/RJ351071.html

我還以為是要督封面那紫色的傢伙 以為是那種對人類仍太早的性癖

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Wait, am I missing something? What and where is the gender change?

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It's one of those "we'll say there's gender change but we'll never show it" type of things. The magical girl is meant to actually be a guy (you can see the use of オレ as a pronoun, for example). The initial encounter with the tentacle monster does something that makes her unable to switch back to her original form.

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I think around page 23 the shota tries to do a saving maneuver, but instead just swaps consciousnesses with his sister.

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