I live for the subtly of relationships.
One of the most obvious subtle things: names and nicknames and general address! Really obvious in anime (that character who's constantly called by their family name or full name except with that one character who uses the given name, good stuff) but outside of that too. In The 1st Rule, Ari never addresses his younger brother in anything other than professional terms, even in life or death situations, but is surprisingly casual with Louis, his boyfriend, and if you read the side story you learn this distance is not a new thing, it's been this sorta one way relationship for year (it's plot important but also he's just way too professional). And also the painful version of this, the K anime, Fushimi is the only one who calls Yata by Misaki, indicating both their former closeness despite working for opposing organizations and getting him riled up because, a, betrayal and b, it's generally a more girly name. And equally, Yata shortens Fushimi's name Saruhiko to Saru, meaning monkey because a, he's mad and b, he knows Fushimi's father picked Saruhiko as his name because he looked like a monkey as a new born (and not in a cute pet name way, he meant it in a asshole way).
How "bantery" can they be? You know, some friends will just insult each other and then laugh. Some can't. Sometimes an insult to your skill is met with a smirk and a friendly match, sometimes it's a serious insult to your honour. Sometimes a bantery, naturally rude character will tone it up or down depending on who they're interacting with to make them more or less comfortable based on how much they know them. In Where The Sky Lies (very soon in the coming updates) Francis points out No 1's only personality trait seems to be he's a liar, and No 1 laughs and agrees, but wouldn't take it from anyone else.
Same with flirtiness (I love a shamelessly flirty character who just gets brushed off by everyone in his friend group and knows when to behave. The Joey, I guess), who comfortable are people with their harmless flirting, do they flirt back with the understanding that they're just friends or whatever, do they get uncomfortable and does the flirt tone it down in response?
Just simple phrases. One of my old works had character A just gently reminding B, their best friend, every now and again that "you're still my favourite person" even as situations around them changed. Another had their reincarnated soul mate just reminding them "don't panic" even when they barely knew each other. And in jokes, around a certain group of my friends "never ever" is met with a bunch of people laughing "except that one time". People's reactions to in jokes let you know how in they are with a group even if the reader doesn't get it and just like with nicknames above, if someone leaves the group it can then get painful. Your enemy is now sneering your own in jokes back at you, ruining them and letting the reader know they used to be friends.