Her Guardian Spirit (Miko X Male Reader)

After the incidents in episode 3 of the anime Miko was walking home upset and annoyed about what happened today. Just wishing she could stop seeing these horrible Apparitions that now haunt her everyday life, but on her way home that day she turned into an alleyway to see a bunch of horrible spirits. Just as she was gathering her courage to walk through them to get home a Mysterious Boy welding a katana dropped from the roof tops and started killing all the spirits in the alleyway, she was shocked who was this boy? and where did he come from? and how could he kill them with just a sword?