username is kins but barely talks about kins

Reo and Nagi had communication problems, yes, and those communication problems were over their misconceptions of each other. Nagi was always aware of Reo's flaws — his selfishness and perfectionism — and he based his assumptions of what Reo wanted on the transactional relationship preestablished between them by Reo. Consequently, he assumed that him going on his own to improve with Isagi and Bachira wouldn't be against the status quo, which it obviously was because Reo had grown emotionally attached and had a catastrophic reaction it it because, unlike what Nagi had assumed, he was also Reo's first real friend. Reo, on his part, didn't understand what Nagi needed out of their friendship. That he isn't a plant that'd naturally grow as long as he kept watering it, and that no amount of natural talent would make up for the flaws Nagi was either not seeing in himself or not working to improve. Hence the breakup.

Kaiser and Ness had a typical toxic relationship of one party, Ness, idealising the other to the point of worship, while Kaiser — fully perceiving his place on the pedestal as his rightful spot — viewed Ness as less than. Ness' last dialogue wasn't just a reminder to Kaiser but a realisation of his own — that yes, Kaiser is human, and a very broken one. Kaiser likely would've suffered much less, including the loss against Isagi, had he been willing to acknowledge Ness' true worth. The trauma runs very deep here, and ultimately, this kind of relationship was always going to fall apart, even without Isagi's involvement. There is only so much Ness could have glamourised about Kaiser, only so much Kaiser could've pushed it before Ness cracked under the pressure.

Sae and Rin's case is a bit more complicated, in that I think Sae is very much aware of everything and, for one reason or the other, deliberately keeping Rin's misconceptions going. Rin, on the other hand, lives his life based on two assumptions of Sae: that Sae is wrong (ambiguous), and that he is the end all and be all of life. Personally I think Rin has the unhealthiest attachment here, since he's literally expressed suicide ideation when it comes to Sae and bases every last facet of his personhood around him. The two of them were driven apart because Rin says that Sae is no longer the idealised image he had of him, Sae lashes out about it, and then they seemingly never talk again. We have yet to see why Sae is really keeping Rin away, but it's easy to assume that, either out of disdain or care, he thinks Rin is better off not being in his world. Sae has fully dedicated himself to living up to being "Japan's treasure" and confesses that beyond that he has pretty much nothing else going on for him.

Circling back to Hugo and Loki, you can very much see the same toxic elements present here. Idealisation (Hugo > Loki), glorification/dehumanisation by way of transcendence (Hugo > Loki), a lack of effective communication (Loki > Hugo, but also in general), misconceptions of the self (both?), an obsessive desire to live up to expectations too heavy for someone so young to carry (Loki). The "healthiness" (and functionality) of Hugo and Loki's partnership is predicated on the idea that Loki is incapable of the same failures that Sae, Nagi and Kaiser experienced, therefore these elements won't be elevated into obstacles between them. And this isn't me underestimating him but I mean, he's not. No one really is. And I doubt we got the match with France so early just for them to go their merry way to the finals.

Beyond that, Loki's character is an important element in that he is essentially the embodiment of what the protagonist dreams of: the heart of his team, never questioned, so talented and so skilled it's considered doubtless he could win against anyone. Loki losing a match would make me personally very sad, but it would narratively enrich the story because it asks an even more important question than the one Isagi did two chapters ago: