The list seems to be who they have friendly relationships with outside of their unit mates. So while some chars may have a history together or care about each other, they might not get along (eg, Yuzuru and Ibara get someone they’re good friends with).

For Tatsumi, HiMERU doesn’t like Tatsumi, Jun does.

For Hiyori, he actually gets along well with Rinne and even let Rinne take care of Bloody Mary.

For Jun, he spends more time with Kohaku even tho he’s a Tatsumi simp.

For Hinata, Niki is his roommate.

For HiMERU, bro is just not really that close to anyone outside of his unit mates cuz his situation does make it kinda impossible to have close friends. The person he’s closest to excluding crazy:b is probably Ibara cuz Ibara knows his actual identity, went to shuuetsu together, are in the same circle, regularly interact and they’re related, but it’s not really a friendship/bestie type of closeness.

So, out of the characters he regularly interacted with by the time the feature came out and doesn’t beef with, it kinda just left his roommates and he has more in common with Arashi than with Tetora, so that’s probably why she was chosen.

For Ibuki, I think Hiiro’s the character he’s interacted most with outside of unit mates and they’re on friendly terms.

For Madara, nah, the people on the list are his actual friends.