Oh my what an interesting little show is being broadcasted here. You can call me Warden boyo, to what you really don't want to know, but I can take a few minutes to explain somethings to you so you don't wind up like your father. First off, there is a Priest in this city known as Kirei Kotomine, he is Rin's Legal Guardian, a surviving Master from the Grail War your father fought in, the man who killed her Father, and the most dangerous man that your father has ever faced. He is an Executor of the Church, a man forged into a Weapon to defend humanity, but there is nothing human left about that creature save for his appearance. A Heart of Evil beats within his chest, a replacement for the heart your father destroyed in what he thought was their final battle, and even without it he is a Broken Existence. Just as you can only feel joy from helping others, and making them happy, he is an existence that can only be happy from the suffering and pain of those around him. He delights in causing doubt and despair with his words, breaking the wills of the pure, and murdering those who put their trust in him. Do not ever turn on your back on this man, Shirou Emiya, for you never know when he will plant a dagger there. Just as he did with poor, poor, Tokiomi Tohsaka.
But, even though he is a monster, you can not face him as you are now. He is a Trained Weapon, a master of martial arts, with speed, skill, and strength bordering the superhuman, and decades of experience fighting those Beyond Humanity, not even mentioning The King that protects him as one would a pet. More so, he is an incredible skilled magus when it comes to healing, a power known as Spiritual Surgery, and he is your one and only hope of saving Sakura Matou from her cursed fate.....my prisoner is stirring so my time grows short. Listen to these final words boy, engrave them into your mind, for they are the first of many keys that will lead to both your salvation and your destruction. I Am The Bone Of My Sword.