One of the Kobizen school swords crafted by Bizen Tomonari. Due to air trapped inside the sword's metal, the blade had many blisters, rendering it useless in battle. Before its presentation to the emperor, they had to fix Uguisumaru in order to make it more presentable. It was since regarded as a work of art rather than something to be used in battle. It was first in the posession of the Ashikaga family, and after Asikaga Yoshinori's awarding it to the Ogasawara family for their participation in the Yuuki Kasen, has been passed down the Ogasawara family line for generations before being given to the Baron Sou family.
Before Ookanehira was added into the game, fans took Uguisumaru's mentions of him as Uguisumaru either having an imaginary friend, the other touken danshi thinking he has an imaginary friend, or Ugisumaru being so lonely around the other brother figures like Ichigo Hitofuri and Kousetsu Samonji that he talks about Ookanehira more to fit in with them.
