You can't defeat him in combat

Fighting against a doppelganger is like fighting yourself. You can’t defeat yourself. Even if you get lucky and manage to do the impossible, it’s a fight that would leave you too weak to continue your travels. It is not something you can afford with a hunter daemon coming after you. The daemon would enslave you, take your freedom away and you’d certainly fail in your mission. But you can’t really just hand yourself on a silver platter. There had to be some way to come out on top.
You put the sword of Saint Magdalena back in its scabbard.

“Fine,” You say. “You can have me.”

“Good choice.” You male mirror image says as he puts his own version of the holy relic away. “You can start by offering yourself.”

Begrudgingly, you sit down on the forest floor and spread your legs apart. The doppelganger kneels between them, places a hand on your quim and starts rubbing it. You carefully take not of how he sits and where he offers some weak spots in his defense.

“You look fine, girl.” The doppelganger compliments you. “When I first saw you bathing in this river, I just knew I had to have you. Such wild hair, those round breasts. You’ll make a good mother.”

You blush and say “Wow, you must be pretty excited.”

The doppelganger sits up straight and pushes his hips forward. “I am, babe. Just look at how big you made my dick grow.”

His dick stands proud, like it is the most magnificent one that had ever been created. You have to admit, you are quite impressed with his member. However you are a sister of the burning rose, so no matter how great it is, you can’t give in to its wonder.

Your reach out for the duplicate’s cock and once you got a good grip of it, you give it a nasty twist.
The doppelganger screams in pain, then falls over. You blink, and see he has reverted to your feminine looks. No matter, you now have your hand free again. You jump onto your duplicate and hold one of your hands on her shoulder, pinning her to the forest floor. Then you ball your other hand into a fist, and start pounding her face.
Beating her did not feel incredibly comfortable for your first, but you’re quite sure feeling a metal gauntlet to her face is far less comfortable for her.

Suddenly the doppelganger yells “I give up! I surrender! You win! Please stop!”

You hold your fist back but keep ready to strike again at a moment’s notice. You look upon the doppelganger and see her crying. There is a cut on her face, above her eyebrow and her nose is bleeding.

“You win” The doppelganger snivels. “I’m sorry. Please stop hurting me.”

This creature probably took your more familiar feminine shape to try and garner sympathy from you. To your great dismay, it works.

This creature tried to **** you. To take away your chastity, destroy any hope of you becoming a paladin, and even attempted to **** you into motherhood. You should beat her to ****. Yet, when you see your image bloodied and crying beneath you, you can’t help but feel you’ve done something every paladin should be ashamed of.

You grab the doppelganger by her hair and **** her to look into your eyes.

“You think you can get away with it just for saying sorry?! You tried to **** me!!!” You yell at her.

The doppelganger closes her eyes, yet It doesn’t stop the tears from flowing.

“Please, I may very well be the last doppelganger in the world. I know it’s wrong, but I’m ****. Wouldn’t you be if you thought you were the last human on the world?”

“Then you should find a person who WANTS to help you, not **** it onto her.” You yell back.

“I’m sorry.” The doppelganger replies once more.

You sigh. This is not getting you anywhere. But then you get an idea.

“You want to make up for it? Then how about this. You accompany me to the high abbey in Gerlangen and when that’s done, I will help you find someone who wants to be with you and become the mother of your children.”

Your duplicate opens her tear filled eyes. “You … you really mean that?”

You relax your features and calmly answer. “Yes, I swear.”

The you shift your expression to anger again. “But if you dare **** me again even once, and I will show no mercy. Understood?”

The doppelganger, seemingly having lost her voice, simply nods.

You get off your duplicate and allow her to rise to her feet. When you ask her how to get to the victory road, she says “Well, there is a bridge downstream. If that is not the victory road, than it probably leads to it. At the very least it would bring you to a human settlement.”

“Very well.” You tell the doppelganger. “Take point.”

The doppelganger starts walking downstream, and you follow her. You just hope she won’t somehow betray you in the end.