well I don’t really like to view Kaoru or Kyoya as a totally sad pathetic little guy. it can be fun of course, but if we’re staying true to canon, I don’t think either of them are deeply sad or lonely because portraying them like that would disregard the strength of the friendships they have in their group. friendship is an incredibly nourishing thing for them that is so often undervalued and underestimated. I think they are both genuinely happy people thanks to their friendships, despite whatever childhood/familial trauma they have.
but I do think some negative feelings would occur, as a natural response to what happens to them in the series. Kaoru let go of whatever feelings he had for Haruhi, regardless of whether we take him at his word (when he told Honey that his feelings for Haruhi weren’t strong enough to translate into jealousy), that must’ve felt upsetting to some degree. and the same is strongly implied for Kyoya, who also told Honey (designated keeper of kyo+kao’s feelings and secrets) that he found something much more valuable that he’s not willing to jeopardize — implying that he loves Haruhi/Tamaki/both of them too much to act on whatever feelings he may have for Haruhi/Tamaki/both of them. that must’ve also hurt to some degree.
knowing this, I think they would at some point come to realize that the things they both love about Tamaki and Haruhi are also things that they could find in each other. I think Kaoru likes Haruhi because she is keen and observant (she sees him as an individual and recognized the ways he is different from Hikaru), she is straightforward (she calls it like it is), super practical and hardworking, and cute as hell.
what Kaoru may or may not realize is that he himself is also keen and observant (understands their friend group dynamics before Hikaru, Tamaki, and Haruhi do), straightforward (not afraid to be blunt), hardworking (one of the top three students in his class), and cute. he knows Kyoya is highly observant and hardworking.
and he starts to see how Kyoya can be passionate like Tamaki — even if he doesn’t show it often — and cute (during the sports festival arc). Kaoru is also like Tamaki in that he enjoys performance and theatrics (he likes making girls swoon), is playful and hyperactive (needs constant activities to not be bored), and keeps the energy high with silly antics.
Haruhi couldn’t move Alice forward in the Alice & Wonderland manga chapter because she lacked curiosity; she had to be thrown into the midst of the story after the twins had followed Honey down the rabbit hole and Tamaki had established a story objective. Tamaki and the twins are similar in that way; but we also have to remember that Kaoru acts differently alone than he does with Hikaru. when Kaoru acts alone, we see that he is the main curiosity driver (digging into Suoh empire lore) and cares enough to move a story forward. Kyoya also does the same thing on his trip to France (searching for Tamaki’s mother) because he cared enough to try… you could say the author used both of them as plot devices, but then why Kaoru and Kyoya in particular? why not Hikaru or Haruhi?
so yeah, I don’t think kyokao is so far fetched from canon. Kyoya and Kaoru may have been “used” as plot devices, but there is enough textual evidence to show how their characters have potential to drive their own stories. and how those stories could very much overlap because friendship and romance work closely together in the ouranverse.
