Kou is Hifumi's superior. Both are bad at conversation and didn't interact with each other for a really long time until Kou called her. Kou makes Hifumi the Character Team Leader so that Hifumi admits that she trust Kou. She sees Kou as someone who is very dense.
Rin is Hifumi's superior. Being observant, Hifumi knows about Rin's infatuation on Kou and the blatant jealously when Kou started talking to her.
Aoba is Hifumi's co-worker. She feels more comfortable when she stays with Aoba. In addition, she have became more talkative and trying to change her shyness since Aoba started working at Eagle Jump. The two are close.
Hifumi and Umiko don't really talk with each other until Hifumi replaces Kou as the Character Team Leader. Hifumi is wary of Umiko and feels like it is hard to talk to her and she even fearfully asks Kou if she needed to talk to Umiko in meetings.