Even with Only Embers of Memory, I’ll Fight the Demon King!

The title is Ashes of Memory Must Fight the Demon King Too, Identity V: Missing (Cover). Since the book is already contracted and the title can't be changed, I'll keep it for now.

A man bought an apple from a merchant. Curious, he took a bite, only to be reincarnated in another world. Forced to sign a contract, he can't return to his original world unless he transforms into a magical girl and saves the world. Now he's really in for it.