Fai always smiles. Not everyone can see which smiles are fake and which are real, but Fai keeps an eye on the ones he feels have figured him out. Those could be dangerous to him. Even more so now that he's traveling with regular companions.
Sakura is no threat; she's innocent enough to believe in all his smiles. Mokona knows, but doesn't pry. Syaoran is the same.
But Kurogane... it's Kurogane who gets to Fai. Because Kurogane knows, and sometimes he corners Fai and demands that he stop. He hates Fai's fake smiles. "Smile honestly," he says. Sometimes, Fai does.
Kurogane smiles in many different ways. Fai once saw him practicing with Sōhi, on a sunny day in one of the calmer worlds they arrived at. Stripped bare to the waist, sweat running off him in rivulets, the ninja had smiled a self-satisfied smirk when he was finished.
Kurogane often wears a predatory smile, whenever he is engaged in physical combat, or planning a strategic move that will save them all.
Kurogane sometimes smiles tenderly, during quiet, peaceful moments, and Fai knows that he is thinking about his princess, Tomoyo.
And sometimes, when Fai smiles honestly, then Kurogane does too.