
Ch.55 spoiler
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I'm not sure either because it hasn't been explained so far but to me it looked like his father's spirit is permanently hiding out inside Ashiya or at least they have a permanent connection. His father fell asleep at the end of the chapter so I guess Ashiya wasn't just possessed.

I think Ashiya's influence is actually Sakae's, as Ashiya doesn't have a full control of his power unlike Abeno. There's this thing about Ashiya being like someone else when he use the power too.. so it's safe to say that Sakae possessed Ashiya only when Ashiya is faced with great danger that could kill him and people around him. Maybe it's related to how Sakae died? Judging by how secretive Aoi to hide Sakae's identity, I think some demons hunted Sakae because of his influence.
Spoiler question for chapter 54....
Is Ashiya being possessed by his father temporarily or is his father permenantly hiding out inside Ashiya? I understood it as the former, but that wouldn’t make sense unless Sakae’s spirit was just hanging around or Ashiya is a medium who spirits can possess from the spirit world or something. Also, is Ashiya’s spirit powers just his father’s, or does Ashiya have power of his own? My Japanese is minimal, so I’m not sure if any of this was explained in the current chapter.