i think one of Shu's core traits is how outside his relationship with Hinako, he doesn't really maintain other friendships.
We know how much Hinako means to Shu. We know he would do anything to help her no matter what. We know he loves her and wants her to be happy. They're partners, members of the Space Army since childhood. He even makes medicine for her headaches and give her a safe space where she can vent about her family troubles.
By all accounts, Shu is a really good friend to Hinako. He accomodates her, he supports her, he reaffirms who Shimizu Hinako is.
But then we see his interactions with Sakuko and Rinko, who he's known since middle school, and he barely acknowledges them. When Sakuko and Rinko show up at Chizuruya, all three girls are happily bantering and teasing eachother as friends. Shu's the only one who doesn't interact with the group at all, not even giving a passing greeting.
It's very telling as well that in the Dark Shrine segments, Rinko and Sakuko's documents mention not just Hinako but eachother, reflecting that they all felt connected in their own ways. Shu's documents, however, only mention Hinako. Rinko and Sakuko are never mentioned once. In his "Dark Shrine" section all his most precious possessions are things relating to Hinako, and he notes himself he'd only let her touch them.
When Hinako points to Sakuko's body in visible grief, Shu glances over but he doesn't comment on Sakuko at all. He doesn't even show any visible signs he's sad about her death (ex. looking down, blinking multiple times to push away tears, trembling lips, shallow breaths, etc.) Shu doesn't do any of that. He doesn't even mutter anything to himself like "poor Sakuko..." It's like she's a stranger to him.
Instead, he's more focused on how distraught Hinako is.
Instead of giving a moment to grieve Sakuko, Shu redirects the conversation to being about the strange situation they're in. One of the first things Shu says is how "even if no one else is left in this world, it's good you're still around, partner". Shu is fine living in a world where it's only him and Hinako.
But Hinako doesn't feel that way. She immediately replies by asking him where Rinko is.
This is one of the few times Hinako gets visibly mad at Shu, and it's because he abandoned one of their other friends.
Hinako's world isn't limited to just the two of them. She deeply values her friendships with her other friends too. It's only after Hinako scolds him for ditching Rinko that Shu now focuses on "meeting up with Rinko" from that point on. He doesn't decide this bc he was worried for her, but bc Hinako was. Sakuko and Rinko don't exist in Shu's "world", and I think that's one of the key differences between Shu and Hinako.
For Shu, Hinako is his only partner. It's always been the two of them, and as long as just the two of them understand eachother, everything will be fine.
It's especially interesting when you consider Rinko is the one who originally invited Shu to Chizuruya in the first place, but he doesn't even say hi to her there. When they meet up with her, Shu avoids talking with her beyond casual pleasantries, and is dismissive whenever she brings up Hinako's "death". Rinko's documents also say that when Hinako "no longer plays with him," rather than finding a different activity or someone else to play with, Shu plays alone.
Not to say Shu has to open up to Rinko if he doesn't want to, but the fact he doesn't have anyone else to talk to sure explains why his "Dark Shrine" self is suspended to the ceiling by heavy metal chains, and suffocates when Hinako leaves him behind.
It's an interesting writing choice to give this "I only need you in my life, no one else" trait to Shu. It's a fun interpretation of the "loyal childhood friend," where his earnest and dedicated attitude can seem great for the partner he centers his life around, but isolates him from everyone else. It's prevented him from forming closer connections with other ppl, but it also makes him very dependent on Hinako.
One of the notable aspects of Shu and Hinako's friendship is how conflict-avoidant he is. Shu is dependent on Hinako because she's the only real relationship in his life. As a result, he is very cautious about doing/saying anything that could potentially hurt Hinako's opinion of him. This can be seen in their very first scene where they sit together in awkward silence and avoid discussing the "elephant in the room" (her arranged marriage). They're both clearly bothered by it, but they also are afraid to address it.
Despite them being longtime friends, they avoid talking about difficult subjects about the "future", and Shu himself refrains from giving Hinako advice besides reaffirming who "Shimizu Hinako" is, or saying he'll always support her. When Hinako first asked him for advice about the marriage proposal, Shu immediately steps back and tells Hinako to "think about it for herself". From Hinako's pov, I can see why it might be frustrating to be told this when what you're seeking is someone else's perspective on it.
But rather than take a risk and say "Don't get married!" or ask "Do you even like Kotoyuki?", Shu is more afraid of disagreeing with Hinako. Or worse, the possibility he doesn't know Hinako as well as he thought he did.
This is also why Shu secretly gives Hinako the red capsules hoping it will "change her mind" rather than talking to her. Because Shu will not take any action that risks damaging how Hinako sees him. He is afraid of taking responsibility for his actions/words, because then it means willingly placing himself in a vulnerable position. It's why Shu phrases the red capsules as being "for Hinako's sake" rather than it being his own decision. He phrases it as Hinako's "choice", but the truth is he wants her to agree with him, because "his partner" would understand him.
I wonder if this is partly what makes "Fox Wets Its Tail" actually rather depressing, and why Shu is missing for "Ebisugaoka in Silence". 🤔
FWIT can be interpreted as Shu's "happy ending," where its only him and Hinako left together in Ebisugaoka while everyone else is gone, including Sakuko and Rinko (Fog Sakuko still being "dead" while Fog Rinko is still "missing"). He doesn't Have to expand his world, because Hinako is all he needs. But the result is Shu never confesses to the red capsules nor admits he's in love with her. He never acknowledges his decisions here, attributing everything solely to Hinako having courage. He stays as her always-agreeable friend, always supporting her without any conflicts, but never having the courage to take responsibility.
On the other hand, Shu not being present in EiS could hint he's stepping back and learning to live in a world not just limited to the two of them. Him saying "I can't expect your help anymore" in his letter acknowledges that he's been dependent on Hinako bc she's been his only close friendship all this time.
Shu isn't incapable of forming connections with other people. After all, he starts becoming friends with Kotoyuki in both neutral endings. Shu's world can expand too, as long as he has the willpower to take that step. Perhaps him being "gone" means he went to wherever Rinko and Sakuko went after SHf. To the future he now has the courage to face, while having the security and trust that he and Hinako will always be partners, no matter what. At least I think so.