Tell the Wind and Fire by Sarah Rees Brennan

Carwyn, but my hold on him changed all the same. Suddenly I was clinging. We both knew that out of everybody in the club, we were the ones in the most danger in a raid. Someone found with dust would only be put in jail.
    If Carwyn was found, the Strykers’ secret was out. If he was found uncollared, with the evidence that I had done it a suddenly heavy weight in my back pocket, we were both dead. I could see from one look at Carwyn that he knew all that as well as I did.
    “Come on,” I said to his sharp, intent face, like that of a hunted fox ready to bite. “This way!”
    We went running through the crowd, elbowing the panicked mob aside. I thought we knocked a few people over as we went, but it didn’t matter. I could hear loud voices in the first room and bodies hitting the floor, and I barreled toward my target destination.
    “The men’s bathroom?” Carwyn demanded. “You’re a very surprising person, Lucie.”
    “You’re a very irritating person, Carwyn. Nadiya’s brother’s friend, he used to have dust on him sometimes when the place was raided. I heard him say there was a window you could get out of.”
    It was a slim chance. But it was our only chance, and I was grabbing it.
    We hurtled through the grubby bathroom door, paint splitting as if the wood beneath was trying to get out. A boy was at a urinal, doing up his fly. He looked at us, eyes wide and startled. His mouth hung wide too; he closed it and then opened it again, as if making the decision about whether to call out.
    I knew that a guard could come through the door at any moment.
    I threw a flash of Light magic at him, knocking him back for a precious instant.
    “Tell him that he can’t see us,” I told Carwyn.
    “I thought you grew up in the Dark city,” Carwyn snapped. “How do you not have the faintest idea how Dark magic works? I could maybe persuade him he didn’t see us if we were running past him in the dark, but he’s looking right at us!”
    “I know how Dark magic works,” I said, and took a deep breath. I turned my hand in the link I had made for us, to hold Carwyn’s hand. “I know secrets in the dark nobody ever told you, doppelganger. Tell him that he can’t see us.”
    Carwyn looked ready to argue, but first he glanced at the boy.
    “You,” the boy began, “aren’t you . . .?”
    Carwyn sighed, closed his eyes, and rolled his neck, as if working out a kink. When his eyes opened, they were covered with darkness, as if under a film of oil.
    “You can’t see us,” he murmured, and there was a thicker sound to his voice. It made me think of blood.
    “Keep saying it,” I whispered.
    “You can’t see us,” Carwyn murmured. I lifted my free hand and my rings blazed, bright enough to blind.
    Something about the air changed. The boy’s expression changed too, blanking out.
    “You can’t see us. You can’t see us,” Carwyn chanted. “Lucie, he really can’t see us!”
    Dark magic affected thoughts and emotions, and Light magic affected the physical world, created energy, and made everything work. I could blind someone only for an instant; Carwyn could make someone believe him, against the evidence of their own eyes, only for a moment.
    But working together, it was different. If you could trick the eyes and the mind both, everything was different.
    My aunt and I had done it for years in secret. I did not like to think about what Aunt Leila would have done if she knew I had shared this knowledge with a doppelganger. I did not want to think about what he might do with it. We had to escape.
    We shoved the boy, blind and stumbling, out through the bathroom door. He thought he was moving of his own volition. He would slow down the guards, and he would not remember us.
    The bathroom window turned out to be real, but small and so high up that we’d have to stand on the top of a toilet to get out. And only one of us could possibly get out at a time.
    We stood there for a minute, on the cracked white floor tiles.