Sukisho! What is their relationship?

Sora and Sunao start out as a bickering not-quite-yet-couple, while Yoru and Ran are an attached, stable one. Yoru and Ran were created to endure Sora and Sunao's hardships, and they've obviously been through quite a lot together, which is part of why they care for each other so much. Ran even mentions in an episode that he's Yoru's most important posession, which Yoru obviously agrees with, but I have the feeling vice versa applies.

As stated about, by the end of Sukisho, we find out that Yoru and Ran are not actually people, but just dissociative identities of Sora and Sunao. Maybe in purest terms, they are the epitome of affection between their hosts. Yoru and Ran seem to love sex maybe more than they love each other, but when it boils down, I think that sex is just part of their true emotions.

At the last scene, Yoru and Ran get one last glance at each other before they disappear. Why? Because Sora and Sunao don't need them anymore. But the expressions they had... you could tell how much they truly did love each other. Maybe there is no happily ever after for them, but their love for each other is forever.

By the end, Sora and Sunao are able to patch up their relationship and get together. Because they are finally able to express their affections for each other, Yoru and Ran are no longer needed to relay and show each other what they could not before.