It doesn't hurt.
This is hardly a revelation. It never hurts. It's times like this, painlessly watching the blood ooze from her forearm, that she wonders if she should exist. She already knows it wouldn't hurt to die.
"If it hurts you, then teach me," Yamato tells her, snapping her out of her reverie.
For the first time, Kouya screams in pain, and the word that gives her voice is Yamato.
Neither notices the slight sweat on Yamato's brow. "My wrist doesn't feel pain either."
Later someone would wonder, "Where does the pain go?" and it would be their downfall. But Kouya already knows. Perhaps that's why Zero seem to be drawn together even more powerfully than others of the same name; because they need each other the most. It's through Yamato that Kouya gets her first taste of pain, feeling the wound on the other girl's wrist rather than her own.
She feigns ignorance later to be worthy of the name of Zero, but Kouya understands Soubi's penchant for pain, even if only a little. She could have walked away from the only being on Earth with the power to hurt her, but instead found herself clinging ever tighter to that pain, the only thing that seemed to be worth feeling.
The only thing that could hurt worse than being near Yamato would be losing her, and for that she would take a painless death over years of numb life. Even that masochist Soubi can't know true pain the way a Zero can. The worst pain is the one you don't feel.