In City of Ashes, Clary and Jace are forced to kiss one another in front of the Faerie Court. They have had feelings for one another for a while, but learned that they might possibly be brother and sister, so they resolve to stay away from each other until they know the truth. This entire episode happened because Clary was pricked in the finger by a faerie and when Clary sucked the finger, there was a small amount of liquid there from a faerie drink. Because she'd taken sustenance from the Faeries she could not leave unless she paid the price they required.
