The Absolute Growth Plan from a Foolish Demon Lord

This is the story of how the Demon King returns to his original world and turns the Hero into his little sister. [Main plot]

The Hero and the Demon King's encounter [Incest?]

An angel and a demon's forbidden love [Older man and young girl]

Two mysterious boys set off on a journey across the world, one to find what was bestowed upon him, the other to slay a dragon. [Actually, they're both idiots]

What kind of redemption awaits a master and disciple on opposing sides? [Relationship confirmed, what a tangled circle]