Hux, as a rule, is all business. The only things he ever talks about are the First Order, the Finalizer, etc... all his interpersonal relationships are purely practical, with the exception of Kylo Ren. Normally, their relationship is just as we see it in canon - they trade barbs but keep it professional (well Hux does). But in the bedroom, things are completely different - Hux is the neediest, most affectionate sex partner Ren's ever had. Hux sits in his lap, nuzzles him and kisses him tenderly, and the noises he makes - they start as soft moans and progress to uninhibited keening, Hux practically rhapsodising over the way Kylo's cock feels inside him. He even seems to love it when Kylo calls him baby - but when it's over, when Hux has had time to come down from his post-orgasmic glow, Hux goes back to normal again. He's not cold, exactly, but the affection is gone, and that cordial facade slides back into place. Hux doesn't seem to have any residual feelings - he just treats Ren like a comrade, until he decides to politely invite Ren to his quarters once again. And it's not just a quick fuck he's after - they can "make love" for hours - it's like Kylo witnesses the dam of Hux's iron will breaking, letting out the side of him that just needs to be held and kissed and fucked, and maybe even loved. Because Kylo's not sure, but he thinks he might be more than a little besotted with Hux, seeing as he can barely make it through their customary terse exchanges without wanting to reach out and touch him, or maybe shake some understanding into him, or just say anything to acknowledge what they do together. Whenever he sees Hux, looking starched and imperious as always, he can't help but think about how beautiful Hux is when he takes him apart. And another, baser part of him feels incredibly smug - Hux stands up there on the bridge trying to look absolutely infallible, but would they all think if they knew how he wails and begs when he's getting fucked open by Kylo Ren?
Tl:dr Hux is needy and affctionate in the bedroom, Kylo's decidedly more attached. (The actual level of Hux's attachment could be unclear, though.)
There are a lot of ways you could go with this prompt, anything is fine by me!