If you're confused by the overall story, here's my interpretation of events so far:
- Michi was an exorcist and friend of Neo's. (Michi was only ever referred to as Kurotori-san around her this chapter, so she shouldn't know her first name, and Neo using Michi-chan at the end is a sign of their closeness.)
- Somehow, despite Aizawa not showing up to school much, Michi befriends her after entering high school and the two start a little romance.
- They go on a date to the Obon festival in August, where things go wrong. Obon is a holiday heavily associated with the dead and something is likely enshrined on that mountain. Maybe the dark figure you see both on the final page of ch12 and in volume 2's spread of Sayaka holding Kasumi. Neo is somehow involved but regrets not doing enough to help Michi.
- Aizawa dies but Michi is able to seal her as a good ghost at school, creating a strong barrier around it to protect her. This comes with the cost of sealing her own memories as an exorcist (or they're simply stolen from her) returning her to a helpless girl scared of ghosts while possibly splitting her powers into a separate entity? (Neo couldn't have sealed Aizawa if this seems to be her first visit to Koga High, and we've already seen Michi's younger self manipulate the rules of the barrier.)
- Months later, Neo returns to check on Aizawa, having heard she's turning into a bad ghost. Something about the mark or split between Michi's current and younger self makes her reconsider Aizawa isn't beyond saving.
- Michi is now charged with protecting Aizawa, since the seal's powers seem to be failing, while Neo investigates the mountain where all this started.
The big question then is why all this was necessary. I doubt it was an accident Aizawa was killed - she was likely specifically targeted - and she has some unique properties as a ghost that don't seem to be a result of the sealing, like getting stronger by the week. Is she supposed to turn into a vengeful spirit and that's what Michi's stopping? Or did Michi simply not want to lose her girlfriend? And is there hope for a more permanent happy solution if Neo is willing to let Aizawa stay for now?
It's also a huge tangled mess how Michi's exorcist powers work and if it's her own autonomous defenses that are wiping her memories when she tries talking to Aizawa. Basically we're at a big jumping off point in the story. It might not move quickly from here, but there's a concrete plot taking shape to be unraveled. A lot of the context will probably change but I think this series of events is pretty established.
Thanks, it's been a while since I saw the optometrist. Sayaka has fuzzy irises too, but I loved that panel of Michi and Neo's eyes juxtaposed so I went with it as-is. They're the only 3 as far as I can tell, and they've had them consistently since the start.