To love is to destroy, and to be loved is to be the one destroyed.

I saw Jonathan after he faced the fear demon, you know. It showed itself to him as you. That told me all I needed to know. The greatest fear in Jonathan’s life is the love he feels for his sister.

For the first time Valentine glanced down at the body of Brother Jeremiah. "I did kill him, and the rest of the Silent Brothers as well. I had to. They had something I needed."
"What? A sense of decency?

But I don't believe I see Valentine Morgenstern. I hear he has charisma enough to draw birds out of trees and convince them to live under the sea, is tall, devastatingly handsome, and has white-blond hair. None of you fits that discription." Magnus paused. "And you don't have white-blonde hair either.

The greatest fear of Jonathan is the love he feels for his sister.

You are interfering in my business, warlock."
Magnus spat blood into his face. "You are torturing a child in my city, Shadowhunter. [...] I thought we were playing a game where we said what the other person was and what we were doing." Magnus told him. "Did I get it wrong? Can I guess again? are you breaking your own sacred Laws, asshole?