Yamaguchi calls Tsukki’s older brother by his first name and it ends it with -kun (not the formal -san). Akiteru calls Yamaguchi by his first name instead of his last name with a term of closeness (like Yamaguchi-kun).
So….It’s canon that Yamaguchi’s been over to Tsukki’s house or been with his family A LOT over the several years they’ve known each other because of the way Yamaguchi and Akiteru refer to each other aka they’re close enough to call each other by their first name (at least Akiteru calling Yams by his first name)
Basically, Tsukishima and Yamaguchi are very close
I realize how much tsukishima hates asking for things. He hates asking for anything. It’s like a mix of him believing that people won’t react well to him asking them and being too embarrassed/prideful to ask. The fact that he hates approaching people and hates needing help probably also has a bit to do with it. Tsukishima was embarrassed and hesitant about asking his volleyball coach for help with volleyball. He waited 6 games and a set before even tentatively asking for permission to time the blocks. He asked hinata to spike for him and immediately followed it up with “im not twisting your arm so you don’t have to or anything” as if hinata, of all people, was going to be offended about being asked to practice some more. He had to ask permission to ask bokuto and kuroo a question. And then was completely taken aback when permission was granted like? It’s a single question? You really don’t even need permission, just ask? Literally any time he’s asking anyone for anything, he’s as awkward as he can possibly be about it. He’s visibly nervous too, he’s hunched over and twiddling with his thumbs more often than not. He honestly acts like he 100% believes that they’ll all refuse to even hear him out.
(PS: the only tsukishima’s shown confidently approaching someone with a request was with akiteru when he was younger (come show me some moves” “can I come to your games?” “why not?”) So… do with that what you will)