The reason I like kabumisu boils down to the way Kabru is set up as a character, then the way Mithrun is introduced, and how the two are forced to interact.
My silly little thoughts under the cut.
Kabru is set up to be someone who isn't honest because he doesn't feel he can be, he holds his cards close to his chest and always has a knife hidden somewhere even in the most seemingly benign interactions. He's deeply traumatised and his sense of threat is HIGH. The higher his sense of threat the more he masks and pretends. He reads people then manipulates them based on what he knows of them i.e. feigning interest and a lack of disgust in monsters/eating monsters with Laius. I think he's even like this with Rin, he seems to know she likes him and he's flirtier with her despite his relationships page saying he sees her as a sister.
So we can establish that his guardedness and manipulativeness is there even with those he clearly cares about a lot. If it was a choice between fight, flight, freeze or fawn, Kabru's natural inclination is fawn. It's a great set up and he's a fascinating character who I'd love to analyse more.
When the elves arrive and we see how Kabru dislikes them because he doesn't like how they treat shortlived races. He doesn't want them swooping in and take the dungeon from tall-men because that's what they always do and they always treat humans like children.
When we meet the canaries we actually get to see his point proven because everyone finds out he's Milsiril's adopted son and are like "oh wow you're so big now, you want some cake?!?!?!?"
Additionally when he says that the dungeon is in the nutrition stage of it's development he gets called crazy and treated like he doesn't know what he's talking about.
However this is not the case with Mithrun.
Mithrun knows Kabru is right and he doesn't treat him like a child, but rather as a source of knowledge about this particular dungeon. His methodical way of approaching things and lack of shits to give means his reaction to Kabru is totally different from everyone else's and he doesn't see him as just a little guy but as an equal, admittedly as a potential tool, but also as a potential threat. He's worthy enough to negotiate with, and there's no patronisation or sense that he's looking down on Kabru just because he's a human.
From Kabru's end, we initially get to see Mithrun through his eyes. Mithrun is a character who continuously subverts expecations; first by taking Kabru seriously as a leader/information source/tool in conquering the dungeon, then again by having 0 sense of direction despite being such an authority figure (this seems to be the biggest one for Kabru at least), then again by having an unexpectedly awesome fighting style... etc.
When they do end up spending time alone together, Mithrun continues to subvert Kabru's expectations of him from the moment he wakes up. He doesn't want to continue fighting him, he's not someone who cares about niceties, and he's pretty vulnerable in unexpected ways despite being strong and great in a fight. As a result we get to see a different side to Kabru; a gentler side, a side where his thoughts and actions add up.
Now this isn't to say that Kabru was never kind or gentle to begin with because he was, he wouldn't be doing what he does if he weren't and he clearly cares for his comrades, shows a lot of concern for the crowds in the dungeon as well as humanity at large, what I mean here is that it becomes a lot more overt and personal when he's with Mithrun. He clearly feels for him, particularly once he learns about his past, and as such he's gentle with him, wrapping him up warm, gently laying him down, telling him to rest easy when he passes out, and thinking inwardly that he'd like to make him something nice to eat. Also he massages Mithrun's feet instead of just learning the sleep spell and casting it on him. What's more he looks after Mithrun SO well that even Lycion comments that Mithrun's hair is looking glossier than usual when they reunite after a week of Mithrun being under Kabru's care.
Kabru's people skills become something really important here, he doesn't treat Mithrun as a burden and he puts his skills to use in helping him and caring for him.
Ultimately over the course of their time together Kabru and Mithrun learn that they want the same thing. That's why Mithrun decides to go with Kabru instead of just going back to the canaries, because he recognised Kabru from the beginning as an equal and someone helpful. Eventually he even sees him as someone pleasant to spend time with, which is why I think he smiles when saying he'll go with him. He could have been more impersonal and pragmatic, but he asks Kabru what he wants (partially because he probably already knows) then smiles and says that's what they'll do. It's more personable.
This commonground and equality is also why Kabru continues to care about Mithrun even after he stops needing to take care of him to guarantee the survival of the others from the threat of the canaries. And it's why Kabru doesn't give up on Mithrun and stokes new desires for him once the winged lion has been defeated. Kabru never saw Mithrun as a lost cause, and Mithrun never saw Kabru as a harmless little human. They have mutual respect, they see each other the way they need to be seen on a fundamental level, and it's great for both of them whether you see them as romantic or not. They play a huge part in each other's development over the course of the manga.
