Because I saw the story wrapped up and peeked at the end.
At some point Ryou sneaks a kiss off Saki’s lips while she sleeps, and has so much life force drained from him, he becomes withered and collapses after seeing himself in the bathroom. She gives back the energy, weakening herself, and runs away. I assume she didn’t want to take away Ryou’s dreams, his life, his future, and she resigned herself to expiring in some dark alley way.
Ryou eventually finds her.
Meanwhile in the underworld or whatever, Saki’s parents watch. The dad says something, but the mom is amused, or proud that over their time cohabiting with each other, Ryou has become fascinated or seduced by Saki, and that this is her answer, and that she’s a good succubus.
Back in the alley, Ryou is about to kiss Saki to transfer the energy she needs to live, when the sex-adddicted friend shows up with what I can only assume is an MP potion of sorts, as Saki snatches it, drinks it, and recovers. The duo return to their apartment and resume their everyday life.
Back in heck, Saki’s mom asks that, if Saki’s a full-fledged succubus now, why aren’t they bringing her back? To which the father replied something about Saki’s happiness and a human life being short, or Ryou’s chances of dying... and in the next page, he gets mad that they’re practically flirting and changes her mind. But he doesn’t summon her back regardless.
Later that same day, Ryou brushes his teeth and goes to bed and wakes up to Saki straddling him. After having had a taste of his life force, she is now unable to hold herself back. Blushing, Ryou has some last thoughts before losing consciousness. Which I assume is permanently, since the story ends then and there.
I've peeked ahead myself and surmised pretty much that myself. I'm hoping the volume has some extras that go beyond that, maybe an epilogue, as that last page feels too much like a Bad End. I don't wanna see a potencially lovey dovey human/succubus relationship die off at the start because Saki-Chan becomes all sex crazed, man!
Regarding the end I'm just going to assume it's along the Catherine lines, with the guy becoming half-demon or otherwise bonded. The two are great friends/lovers, and I really don't see one of them dying as a satisfying conclusion to the series. Let the two lazy bastards live in peace .
First glace: "Interesting idea"
First few chapters: "This is gold!"
80 chapters in: "where you go plot?"
That's the basic story. That said, kudos to the translators for explaining the cultural jokes and puns the best they could. Really made sense of the chaos of the manga. It ran but it didn't feel like it was running too long or beating a dead horse. It transitioned from a plotline to 1-2 page gags, then had a strong finish.
Was a fun read while it was being released, but ~300 pages (quite a lot of multi-page chapter extras and translation notes) in a binge might make it seem worse when the story gets parked. I'd like to recommend but I just have to say you may get bored because the plot vanished for 100 pages. If that doesn't bother you, and you just want some laughs, read it!