A/N: No spoilers; this takes place fairly early in the series, chapter 26-27 of the manga and/or anime episode 13 – in Jade Country, their first night in Spirit.
Fai sat down on the edge of Kurogane's bed after Syaoran and Sakura had already gone to sleep. Kurogane was in front of the fire.
"You don't like the cold, Kuro-rin?" Fai asked with a smile, hands on his chin.
"No."
"Hmm... Maybe Kuro-rin could use something to warm him up at night...?" Fai suggested slyly as he glanced out the window. The sky was grey; it looked as though it would snow that night, and the temperature was already dropping fast with the setting of the sun.
"What's that supposed to mean?" Kurogane demanded as he stood up and turned around to face the magician.
"Nothing, nothing!" Fai replied with a grin, and waved his hands.
"Liar. What are you plotting now?"
"Are you going to bed soon?" Fai asked, changing the subject.
"I suppose there's no use staying up if we don't know that anything's going to happen tonight," he grunted as he started getting undressed. "I really hate wearing these strange clothes all the time..."
"They're not so bad," Fai said with another smile as he watched Kurogane take off his shirt. "Besides, I think they look good on you."
"Hmph. All the more reason to get them off, then." And, when he had stripped down, "Now move your ass so I can get under the covers before I freeze."
Fai obliged him with a smile, and started to unbutton his shirt.
"What do you think you're doing? Get out and go to your own room!" Kurogane told him.
"I asked Kuro-rin if he wanted something warm, and he never replied," Fai told the wall. "So I guess that means he doesn't mind, right?"
"W-What?" Kurogane sputtered. "What do you think you're doing, idiot mage?"
Fai finished getting undressed and let his very last item of clothing drop to the floor before turning around and, before Kurogane could utter another word of protest, slipping under the covers and climbing on top of him. "Don't worry, Kuro-sama... I'll warm you right up," he said with a wink and a smile, and let his hand wander down Kurogane's chest.
"What are you doing? Get off of me and out of my bed!"
Fai hushed him with a finger to his lips. "But Kuro-sama doesn't like the cold much, right? So I will..." he trailed off as he let his fingers drift to Kurogane's cheek, and leaned down to kiss him on the lips. Kurogane reached up to try to push him away, but Fai was not so easily budged and Kurogane was distracted to begin with.
"See? That's not so bad, is it?" Fai asked him in a whisper, his cheeks flushed. "I'm feeling warm already... how about you?"
"Idiot... get off me," Kurogane grumbled, even as he hesitated to reach up and slide his hands over Fai's hips – to get better leverage to throw him off, he told himself.
"Maybe you need more convincing...?" Fai suggested with a smile, and kissed him again. Kurogane's fingers tightened against his skin, digging in and pulling him down, but Fai didn't mind – it was what he wanted. Without protest, Kurogane let him kiss his way down to his neck, but when Fai slid his hand down over his stomach and slipped his fingers under the waistband of his boxers, Kurogane decided that was going too far.
"Stop that! Get your hand out of there!" he shouted, and Fai smiled.
"But I'm just getting started," Fai replied, and nibbled on Kurogane's ear as he obediently withdrew his hand. "Better be quieter or someone might hear us."
"Idiot... You..." Kurogane protested through gritted teeth as he tried his best not to give into Fai's ministrations.
"You know, if you'd just enjoy it..." Fai suggested with a smile as he slipped his hand back down and under.
Kurogane couldn't help the moan that escaped his lips, but it was quickly followed by "Damnnit... Fai..."
"Getting warmer, hmm?" With his free hand, Fai tugged Kurogane's boxers off and started to work his way down Kurogane's chest.
"What do you think you're doing now?" Kurogane demanded frantically as Fai wriggled his hips out of his grasp and slipped backwards.
#
When it was over they lay there together under the covers, spent and panting, Fai's hand curled against Kurogane's cheek.
"I suppose you think you're going to stay here," Kurogane said at length, staring up at the ceiling.
"But of course, Kuro-sama," Fai purred as he stroked Kurogane's cheek with a finger. "I said I'd keep you warm, didn't I?"
"Those kids and the doctor had better not have heard us, then," Kurogane grunted as he reached up to touch Fai's hand – he'd meant to pull it away, but didn't.
"Mm... I think you were louder than me," Fai told him with a smile as he brushed his thumb over Kurogane's lips. "But you liked it, right?"
"Hmph," was Kurogane's only reply. Fai smiled, and leaned over to engage him in another kiss, but Kurogane turned his face away. "You can stop that now. I'm going to sleep."
"Goodnight, Kuro-sama," Fai said with a smile, and kissed him on the shoulder instead.
#
Syaoran was looking out the window at the snow in the morning. "The town's covered in snow," Fai said as he came up behind him to look out as well.
"Well, it was freezing last night," Kurogane said as he joined them. Fai gave him a knowing look and smiled, while Kurogane stubbornly looked away and out the window.