Shiiho is a Shinki or Vessel, basically a human spirit who can transform into a God Weapon to destroy an evil spirit.
As a shinki or vessel, Shiiho's name is Tsuiki. Tsuiki takes the form of an army of men on horseback. Shiiho belongs to Arahabaki one of the Emishi God. So Arahabaki who belong Shiiho as a vessel has to call him by his vessel name so Shiiho can form.
Content compiled by AnimeList.moe from publicly available sources.
