Doppelganger

Considering this is now part of the official patch, here we go!

Doppleganger is the secret boss which Rogue added into Monster Girl Quest Chapter 2. A monster which has all of Luka's abilities and no requestable skills, Doppleganger will parry all of Luka's attacks with the ones he uses.

To win, you have to take advantage of her SP count and her habit to use meditation when she's low on health.

If Luka loses the fight, Doppleganger proceeds to rape Luka, impregnate herself, and switch bodies with Luka afterwards. Luka lives the rest of his days as a mother to his child while the doppleganger goes on a demonic rage, destroying everything.