Here, have some Hatoful Boyfriend Fridge Horror. (Thanks again to TV Tropes for giving me this horrible realization ... you guys are the best.)
[massive spoilers]Okay, so, admittedly, I was wrong about the "Yuuya always quietly dies offscreen in every otome route except his" part (he survives in Anghel's route and Sakuya's extended route.) However, now that I've hundred-percent completed this game, it's a bit too late to find evidence to the contrary on this one....
You know the ID of the missing student? The one that you give to Kazuaki if you're on any non-suicidal route (at which point he says "that's strange" and confirms the student in question is missing,) give to Murderbeaks if you're pursuing him, or find out it's actually that of Akagi Yoshio Higure Anghel, the Crimson Angel of Judecca himself, in the Mad Love of a Fallen Angel route?
Conclusion 2: The only ending in the entire game in which no main character dies is Anghel's otome route (unless you think Anghel and the protagonist got high and killed Shuu, in which case none of them qualify. Anghel's ending is a little unclear.)
Edit: Conclusion 3: alsdjfghaldfhgal ANGHEL'S GIBBERING ABOUT BEING FREED FROM THE CAGE OF ICE
MURDERBEAKS HAD HIM IN HIS FRIDGE OR THAT COLD STORAGE UNIT FROM BBL OR SOMETHING
[BBL spoiler]When Anghel attacked Wallenstein in BBL, he was telling the truth and completely justified in his actions; he was attacking the person who kidnapped and tried to murder him before he somehow escaped. No one took him seriously, not even us as the players/audience, because he's Anghel. :(
In regards to Conclusion 2: From what I recall of Anghel's ending, Shuu made a classic, albeit cliche "You have not seen the last of me!" escape from that battle.
Perfectly dramatic and leaves the story open for continued adventures. Knowing Anghel, that'd be precisely how he'd want things to keep going. After all, once the main villain's dead, how's a fallen angel supposed to be able to redeem himself from the frozen cage of Judecca any further?
...Dear lord did I just say that with a straight face? o.O;
Anyway, yeah, I'd say the odds are pretty good Shuu fought, ran away, and lived to eviscerate another day....and probably wondered what the hell happened to him as soon as he was out of the range of Anghel's influence.