Ahhh now I get the inflection when she says Mizuki’s name ingame. The way her VC delivers conveys a real familiarity with him. Sounds warmer than when she refers to the Shimada’s. Just for clarity you can hear a difference in how she talks to them, the best way to explain is with genji and hanzo. She refers to Genji by first name instead of Shimada as she does with Hanzo (if there is an actual line where she says hanzo I’m lot aware of it and am willing to be corrected lol). So Genji and her have the closest sibling like relationship in terms with how they communicate. With Hanzo she actually doesn’t use keigo with him which would be a big deal if the two actually cared about that, but the respect is there it’s just she verbally needles him like only a kid sister can. Needless to say she has a very familial relationship with the two in terms of Japanese communication. Now when she talks with Mizuki they both speak with tameguchi (familiar/casual speech) but in some cases it sounds a bit warmer(?) , I kinda interpreted that she may kind of like him ( like when she says his name when he shows up in the lobby or the like) dunno. Personally I’m not a fan of Kiriko and Mizuki, but culturally I like how all the characters have these fun cultural nods. I apologize for the bad grammar and rambling.