Calling it now: if it's truly from a show, Honami was supposed to be in it and some supernatural shady shenanigans happened behind the scenes which got her killed because the entertainment business isn't nice. Her group mates have nothing to do with it imo.
Also, ok I'll give senpai a break, she's just extremely weird. And probably is lucky that no incident happened to her like whatever happened to Michi that made her fear interacting with the ghosts. I still want her to eventually realize she's seeing a dead girl, there is just no way she can go for too long without noticing, right? It's a dead idol. If she got into their classroom she'd see the picture on the desk, if Michi takes out something Honamin related she'll say "hey isn't that your friend?" and Michi will tell her it's a classmate who died and used to be an idol.
If the senpai notices what she's seeing are ghosts, she might realize that some of them are dangerous, which will make her an ally whenever an incident like the one from Chapter 8 (?) happens to Michi.
To those wondering: this is the symbol. It's creepy. I wonder what kinda show that'd be. It could also be from some supernatural show unrelated to Delphinus but related to the culprit of whatever is happening with Aizawa-san.