I don't think the middle-aged man is really the Grim Reaper. He's presented as something with malicious intent. I think that he's just some old curse or an especially powerful spirit.The grim reaper most likely
The reaper have been following miko since elementary school enterance (or might be even longer). Remember miko didn't have the power until her father passed away, which originally her father power.
So either it inherited to her automatically or her father have her that power on his dying breath, maybe so she have a chance to protect herself from this entity.
This is what vicious mockery actually does.insane insults
Exactly my thought. Yakuza protecting the young miss lolScary, nice, awkward. So they're basically yakuza. The good guys version.