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Miko's father's death was orchestrated by a ghost
At the end of manga chapter 54, Romm shows Miko a picture of a specific ghost that he, apparently, cannot look at again as it might hunt him down. Miko recognized the same ghost in a childhood photo, it's possible that her father also saw spirits the same way Miko does and looked at the ghost enough times to become a target and the traffic accident that killed him was caused by the ghost blocking his vision, possibly while he was driving, since Miko's vision is blocked when she's enveloped by ghosts.- It's yet to be explained how the ghost chooses its targets.
Miko's mother can also see ghosts
Her ability is more limited and is like Yuria's, she can only see the weakest ghosts. That explains early episodes/pages, she can hear her husband cracking jokes but she can't see more powerful ghosts or auras. Consequence of that is that she can't see Horseface ghost or Mitchiru's ghostly tentacles. If the above theory is true, that Miko's father could also see ghosts, it would give Miko talent from both sides of family tree. Miko's family has talent to see ghosts but it manifests during puberty
Unlike Romm and Mitsue, her parents' talents manifested during puberty. It is a protection thing due to evolution. Any kid that could see most powerful ghosts and not have a powerful aura to match (like Romm) for self-defense would get killed before they could be trained, the moment they point they can see the powerful ghost. Yuria can see weakest ghosts and auras, keeping her out of danger. Mitsue can see slightly more and has more powerful aura and powers to match. Romm has strongest vision and aura and that almost caused his death multiple times, including being killed/kidnapped by Shrine God. So their powers manifest during puberty, when they are reasonable enough to know the ghost rules (pretend not to see them, use salt, use charms etc) which a young child would have trouble following. The whole world runs on supernatural ecosystem, with weaker mediums being protected by their inability to see stronger ghosts and thus not becoming a target (mostly), stronger ghosts preying on weaker ghosts and mediums who can see them while stronger mediums can both see and exorcise both weaker and stronger spirits. Its a delicate balance of predator and prey between ghosts and mediums in a supernatural Rock–Paper–Scissors way. The ghost-eating monster in chapters 4 and 13 is actually a Psychopomp.
It "eats" ghosts that have fulfilled their Ghostly Goals or who are otherwise ready to move on to the next world. That's why the spirits lining up before it are mostly humanoid, and why the old man's ghost from chapter 13 was in line the next time Miko saw him.- Seems to be confirmed by chapter 28; while Miko sees spirits lining up to be devoured by a strange monster, Mitsue sees spirits fading off to the afterlife.
The spirit haunting Toono Zen is the ghost of his abusive, controlling mother, and represents her toxic influence that turned him into a creepy animal-killer.
- Confirmed. Though Zen turns out to be a subversion, and he never killed any animals. It was his mother's ghost that drained them of life and killed them.
Shindou Romm is the old fortuneteller's grandson.
She once mentioned that she has a grandson who's about the same age as Miko. Shindou seems to be somewhere from late teens to early twenties so close enough.- Jossed. He's her apprentice.
Romm is Mitsue's FORMER apprentice, because she banished him.
- While Mitsue sees ghosts as still being people, if formerly living, Romm sees them all as hostile monsters that need to be destroyed. This is also why Mitsue does not want Miko or Hana associating with Romm, because he would use them to bring harm to otherwise benign spirits.
Most of the spirits Miko sees are completely harmless.
They're just so desperate for attention (or to resolve their Unfinished Business), it's made them clingy and annoying and desperate to find someone who can see them. Some, on the other hand, are clearly unstable, perhaps dangerously so, and a rare few ghosts are capable of interacting with the mortal world... Miko's ability to see ghosts, is for an important reason.
She gained the ability to see spirits, because she is destined to protect Hana from dangerous ghosts. She just does not know it yet. The Dragon god is simply a very powerful ghost.
Maybe he's simply that much more powerful because he's very, very old or because people worship him. Of course, the implication is that every ghost can theoretically becomes a "god".- Confirmed. Shrine God is a ghost of ritualistically sacrificed shrine maiden.
Miko is not a medium.
Miko is very unlike any of the other spiritualists. Julia, Mitsu and Romm all started seeing ghosts from a very young age, maybe even since they were born. Miko only started seeing them once she was in high school. All other mediums can see life aura as well as ghosts. Miko only sees the ghosts. She's very unlike them, so there must be something else going on with her. Miko will die.
Due to the fact that the Shrine God would be after Hana due to the "flesh and blood" clause, Miko would voluntarily offer herself as payment in order for Hana to be spared. Whether that means that she is permanently dead or it's a temporary one is uncertain.- Jossed. Miko does offer herself to the Shrine God but it is a distraction while Mitsue and Romm do their thing. Miko doesn't die during the encounter and is saved by Romm, then Mitsue and finally Oka Towako.
Miko will lose her ability
Admittedly an obvious theory, but once it is revealed why Miko can see ghosts, it will disappear much to her relief. Alternatively, the more she comprehended her power, she'd become unfazed.
Miko will become unfazed from seeing ghosts and there is also possibility that comprehension will change what she sees (fading into the light as compared to currently seeing the ghost getting eaten). Hana eats a lot because she is incubating an eldritch god.
Would earnestly explain all her hunger pangs and the burst in her aura.
Miko was destined to be a Miko.
If she had been born in a previous century, her power would have been recognized quickly, and she would have been taken to be trained as a full-on Exorcist. But because such things are basically only kept around out of tradition nowadays, she’s stuck simply enduring.