My two main characters are twin souls. One soul is Yang, the other is Yin (Positive/Negative personalities). Both are 15 year-old orphan teenagers sharing the bond of love.
The problem is, I want to cut that bond and turn it into hatred. The Yang soul is the girl, she is grateful to my Yin anti-hero for saving her life.
They run away together from the orphanage and join a martial arts dojo where their sensei takes care of them. He trains both to become exorcists. 5 years later, the sensei dies at the hands of a demigoddess succubus and my characters are the only survivors after a bloody battle. The dying demoness reveals that my Yang protagonist is a demon-human hybrid and that she is one of his many demon half sisters, then she dies. Then both of my protagonists start their journey.
How can I turn my female second protagonist into the antagonist during the journey?
(Note: My Yang protagonist is very possessive of his twin soul. He is misanthropic, quiet and peaceful when not disturbed, but he gets angry easily and enjoys torturing his enemies killing them regardless if the enemy is demon or human. He values justice above all things because bandits killed his family for a relic that they sold to a lesser demon. The female Yin protagonist is the opposite of him. The only thing they have in common is mutual hatred for demons. However she may befriend a demonic prince in the future.)