Names and honorifics
I'm a sucker for how people get called different things (like Fate's 6 names in Nanoha A's, none of which are "Testrossa-san".) I realized there's stuff to mine here, too.
Umi: something about her makes almost everyone jump from 'Koumoto-san' (or '-senpai') straight to 'Umi'; I don't think Shou has ever used 'Umi-chan'. Ibu says 'Koumoto', and a couple of old middle school friends said 'Koumoto-chan' and 'Umi-chan', also Fuki 'Umi-chan' on the very first page, but mostly people are either fully formal or totally informal.
Umi herself is a heavy -chan user, using Nacchan for Nae and Zumi-chan for Zumiko. Switches between 'Shou' and 'Shou-chan', though 'Shou' in her thoughts and their private moments.
Much of the friend group is on a personal name or nickname non-honorific basis. Yosshi (Yoshida) is always just that, Maki is usually Makki (even to Umi). Zumi is usually Zumiko (except for Umi). Nae is often Nae, sometimes Nae-chan or Nacchan. Ibu is Ibu-san or -chan usually, but Yuuki to Yosshi.
Or grouping things by what people use for others:
Umi: mostly -chan, except for 'Shou' and 'Makki', also 'Kosugi' (the first camera girl)
Yosshi: uses 'Umi', 'Yuuki', 'Zumiko', 'Nae', 'Makki' -- I didn't notice a single -chan out of her
Maki: 'Yosshi', 'Zumiko', 'Nae'
Nae: 'Umi', 'Ken-chan', 'Ibu-chan'
Ken: 'Nae', 'Yosshi', 'Makki'
Ibu: 'Koumoto', 'Makki' (once); I didn't note her talking to or about anyone else. Well, 'Hey, glasses' to talk to Baba.
Some interactions just have no evidence, either I missed them or they don't happen. Like, even in the friend group, seems like most girls don't bother talking to Ibu. (Even Nae's 'Ibu-chan' was more "is she okay?") I don't think Maki has been showing talking to Umi.
I'm leaving out the default 'lastname-san', and the teachers, who seem to use 'lastname' or 'lastname-san' at random.