Disclaimer: Yu Yu Hakusho and all related characters are the property of Togashi, Funimation, etc.
Warnings: Dark content. Mention of off-screen character death.
Lines in the Sand
There are lines they don't cross.
Yukina is not an issue that is brought up, except under the most innocuous of conditions. They don't talk about how Kuwabara would have died for her. They never talk about how she was killed.
Likewise, the issue of Kuronue is never discussed. After Yakumo's defeat, Kurama never attempted to explain and Kuwabara knows not to ask.
Kurama does not share a lot of things with his mother. Kuwabara respects that and never suggests otherwise.
Kuwabara shares almost everything with his sister, which Kurama eventually gets used to.
Work is a free-for-all, no holds barred and no forbidden areas. In a fight the only limits are their own, physically, mentally, the limits of their own endurance.
Sex is largely the same, and Kuwabara has, on one or more occasions, made the remark that it probably doesn't say much for them that they can make that comparison. Kurama thrives on it, though. He'd not expected to find a lover in the human world who wouldn't balk at his demonic form, or shy away from an aggressive touch. Humans are perilously delicate, but Kuwabara could break Kurama if he wanted to. It's… invigorating. Kurama doesn't share that observation, because Kuwabara already thinks they're strange enough.
They don't talk about feelings or emotions because they're guys and Kuwabara may be a kitten at heart but he blusters and stammers like any young male when faced with an emotional confrontation. Kurama is fine with that; he far prefers acting on problems to talking about them anyway, and so far the only hurdles in their relationship are from outside forces.
They never, ever, under any circumstances, discuss Urameshi Yuusuke and how he fell from grace.
It hasn't escaped Kurama's observation that all the limits in their relationship are based on loss and grief. It might be healthier if they didn't have those limits, but he can't bring himself to believe that reopening old wounds will help either one of them.
It hasn't stopped them from being good together. Five years have passed since Kurama first tumbled his human teammate in some godforsaken corner of the Makai, with no intention beyond easing both their hearts a little, but somehow they've ended up coming back to one another time and again.