Masaki mentions 'that one girl', a former DeLphi member who isn't Honami. Crackpot theory: it's Micchi, who grew out her bangs and has amnesia
I can't help noticing that after “that one girl” Masaki also starts talking about their auditions.
Maybe there was some girl who has lost her inner composure when they flunked her and thus she did something horrible to herself or to a successful idol. And by “something horrible” I mean the real cause behind one or more of the following things:
① Aizawa's death,
② Aizawa's lingering attachment / regret / desire / whatever,
③ Michi's apparent amnesia about the fateful August / summer festival,
④ the mysterious gathering of malicious spirits around the school
before Aizawa died,
⑤ the ghost that haunts Ange and probably haunts the whole De:Lphinus.
For the latter, I guess, a vengeful spirit of the flunked girl is a good candidate. Or a vengeful spirit of a successful girl who suddenly becomes unsuccessful (and thus very jealous towards the living idols) because she is brutally murdered by the flunked girl in cold blood while the rest escape with their lives and can form an idol group without her. Or…
…by the way, we are not really expected to blindly follow Masaki's words. (She might be an unreliable narrator of past events.) Maybe there have always been two or three such girls, not just one.
Additionally, if we follow the trope of
“Tasogare Otome × Amnesia”, then a pretty likeable amnesiac ghost could have an ugly repulsive knowledgeable shadow. Michi has lost her memories of the summer festival and some of her powers, but her shadow “imouto” (little Michi) seemingly remembers everything. What if Aizawa has lost her voice because she also has some shadow “imouto” who will be perfectly able to speak when we all finally meet her?