Ok this is pure speculation and probably wrong but whatever.
If I understand correctly, in this manga there are two "levers" you can pull to interact with the spiritual, your "power level" and your memories.
Power level example: Horimitsu doing what Kurasawa said and giving up her blood to draw the star symbol.
Memories example: This is way more complex and pretty much all revolves around Michi.
I think Aizawa is a ghost whose form is "pinned" by the gap in Michi's experiences. Michi gave up her experiences of Aizawa AND some of her power to keep Aizawa on earth. This is why Michi can touch Aizawa later in the manga; it's not that Aizawa is becoming more powerful, it's that Michi is becoming more powerful. Not sure if it's just because of the passage of time, or if it's because she's remembering more about Aizawa. But recall that touching spirits is something the spiritually powerful Horimitsu can do, but not the less spiritually power other students.
Emphasis on "the gap", it's a gap in not just in memory but in how Aizawa fit in her life. Aizawa's independent existence relies on the gap's existence; Aizawa IS the gap. Presume that Aizawa used to be her best friend. When Michi asserts they were never friends, Aizawa starts to cease to exist. When she asserts Kasumi is her best friend, Aizawa becomes Kasumi. What Michi asserts (verbally) with regards to her friendships becomes Aizawa's reality.
Time for some wacky stuff.
Aizawa is Michi's gap in knowing Aizawa. Who is mini-Michi? Probably, it's ghost formed from her elementary school experience that made her afraid of ghosts. What's interesting is that mini-Michi has tangible form. Horimitsu can see her.
Ok, who is the ghost haunting Ange? They have long dark hair.
Theory 1. ch. 25, Sou (Michi's mom) has long dark hair. Let's presume the ghost of Michi's mom, hanging around because of her spiritual power, is haunting Ange. Based off Horimitsu's grandma, maybe spiritual power is hereditary? And if Michi is spiritually powerful, maybe her mom was as well?
Why would she be haunting Ange? She wants to kill Aizawa, right? The best motivation I can think of is wanting her daughter to recover the gaps in her memory, hence the "not enough"; Michi should have more friends, but the gaps in her memory are stopping her. But not super convinced. Brings me to the next theory.
Theory 2. Michi ("Mi"zu to "Mi"chi) was the 5th member. ch. 17.5, Michi's idol form would have long hair.
Narrative is like this: Michi's gloominess stemmed from mini-Michi, to protect mini-Michi's gap; people generally won't ask you about your childhood if they aren't your friends.
She meets Aizawa. They become friends. With Michi's athleticism, loud voice, and good looks, she beats the odds and becomes an idol. But "something" happens. This "something" becomes a ghost fueled by a gap in Michi's memory so large it extends beyond her into pretty much everybody, like a star symbol drawn across the whole nation, or maybe broadcast (through TV?) across it.
This ghost is associated with negative emotions. It wants kill Aizawa, then Ange, etc. to take revenge for something they did. Hence the "not enough"; it wants Michi to be more popular, as a superstar idol should. Again not super convinced.
Theory 3. The 5th member is literally a new character. I think this makes the most sense. But the above two theories are wackier, some nice fuel for speculation.
