[Thread Caren Hortensia] Spoilers warning
As usual,this is my own interpretation.
Caren Hortensia,daughter of Kotomine kirei and Claudia Hortensia,comes in Fuyuki as Kirei's replacement as the church's advisor. Having never met her father and her mother committing suicide

She spent her whole life working for the church as a priestess. Because of her ability "masochistic spiritualist disposition",allowing her to feel the same pain when nearby someone possessed by a demon,she was used as a detector.Having to hurt her body in order to help people.

One could say,she even became a demon herself. Unlike her father,searching for one self, she could be described as having two personas.The quiet and shy priestress girl inside the church,playing the organ wearing a blue dress.

The confident and serious exorcist girl ,outside the church ,wearing stockings over pantyhose without a skirt to increase her mobility and seduce men by using the Shroud of Magdalene to bind them.She grew up as an independant girl,always completing her duty.

This time, she has to take care of Angra Mainyu,posseding Emiya Shirou.However, they end up developping a special relationship for one another,of both love and hate. Shirou/Angra feeling sick toward Caren,who is always helping others even if she ends up hurting herself

Reflecting Shirou's personnality of putting others life higher than him,as if he was looking at a mirror of himself,but for Caren,no one was present to refrain her from hurting herself everytime unlike Shirou. Yet,he still goes in and listen to her music everytime,admiring her.

Caren who spent her whole life helping eradicate the demons,is faced with one of special nature. Angra being used as a scapegoat to host all the evil in the world and using Shirou's body,a pure soul seeking justice ; sees Karen in a special light.

A human that he doesn't feel murder toward but more so lust.For Caren she finds a demon she wants to help herself not because someone told her to do it but because she wants to.Supporting both good and bad sides, consumming each other.

The priestess,walking up the stairs accompanies the demon back to the holy grail, like a requiem.The end of a forbidden love that cannot exist.With a final smile,calling him a rock star,which is ironic for Caren who plays religious music.