Hickey
A hickey technically isn't a kiss; it's a red mark (a bruise, really) left on the skin after someone sucks hard enough on it. Hickeys hurt a little to get, but some people think the sucking feels good, especially on the side of the neck. It can be embarrassing to walk around with a hickey, so before you start sucking, get permission first.
Fai's pale skin bruises easily, and many times this has proved a problem for Kurogane. He is not used to being so gentle, and how he craves to go rougher and harder. However much he enjoys the sound of Fai's moans of pleasure, the sigh of Fai's spread thighs and prone body make him wish he could simply give in to passion without hurting the smaller man.
But he hates to see Fai hurt.
He's seen it before, after all. He remembers with far too much clarity how the mage's limp, nearly lifeless body felt in his arms. His eye gouged out and blood running like tears from the empty socket, the only sign Fai was still alive was his ragged breathing. It was no better when Fai finally moved again though. His muscles contracted in spasms as new blood shot through his veins, and his eyes are wide and wild in pain.
Kurogane would rather die than see such a scene again.
On the nights when Fai is feeling especially frisky, he practically begs to be taken roughly, and Kurogane finds it impossible to resist marking him up just a bit. He contents both himself and Fai with kisses and nips on easily covered skin. He sucks at the blonde's chest, his abs, his thighs, knowing full well hickeys will remain the next morning as proof of the past night's activities.
Fai doesn't really mind; Kurogane is the only one who frets over the mage's bruises. For Fai, if it's Kurogane, he can bruise Fai all he wants.