It is not a sin to love a fictional character who is already in her universe. Go on and let yourself be in love with her because after all, she is just your character crush who do not exist in our real world. Everyone of us has their own crush or has their own stories about falling in love maybe for a living thing or non-living thing. All of us have the freedom for it and I can tell, you are just experiencing Fictophilia which means desire, sexual attraction or you have that intense feelings of love towards the fictional character. So, you do not have to worry because you are not alone. Even me, I have a crush for an anime and I know that I'm in love with her. My friends also bought hundred pieces of their favorite fictional character and put it all inside their bedroom. To connect with that, we are also experiencing Fictophilia.
I am one of devout Christians that can say it is really okay and you do not violate the commandments of God. As what you have said, it is not a sin unless you're lusting her or gone in unhealthy level. It will be a sin if it is a real person that may ruin affection to her family that God created in this world.
Sometimes, you love them so much because you know every time you see her and she do her character inside their own universe, you find love, you find peace, you find happiness or you found that perfect love for her which you can't control. And it is okay to idolize a male fictional character even you are also a male and sometimes you pretend to be him and do his scripts like a crazy person. So, keep in mind it does not matter and just make yourself feel better.