I'm not convinced "Mizu" is actually dead, let alone the spirit. Especially as Masaki is an unreliable narrator.
with bangs specially reinforced on the only side we know Mizu had bangs,
and scruffy permed hair that is a bit too opposite Mizu's straight-cut locks
the two are counterparts somehow, and so far just a few months of depression and a hair curler apart
It could be nothing, could be Masaki's downplaying herself around Micchi, a relative, or could be a sign Masaki isn't new to becoming obsessive about people. I feel bad for Honda as her rock. Since Masaki is a path to supernatural elements I'll go on a limb and say she's set up as a victim, maybe as collateral damage, after this arc of her being a potentially violent threat is settled.
There's a lot of potential, but I'm wondering what kind of psychic hole would be left if someone had their fitness or talent ripped from them by a curse... And if they'd act a bit ghastly afterwards. That would be a metaphorical death, from their conscious perspective, and a torn soul is liable to act a bit haunted around psychic Micchi. Too many routes to guess.
Even wilder induction in lieu of deductive evidence: Mind that the first 2-volume plan with Masaki would've had an ending, and probably no more cast than we've seen save spelling out the big bad.
We should have all or nearly all the parts for a complete plot, even if it's now a red herring. In that case it would be fair to have the spiritual villain be part of a main cast member, perhaps severed or digested by a worse monster rather than Mister Hyde-like. I don't think Masaki is the villain, but I think at least originally she would've been very closely connected to Aizawa's killer by virtue of whatever her damage is.
Such a fun series.