According to the Heike Monogatari, Shishiou was a sword given to Minamoto no Yorimasa after he slayed a demon called a nue that cursed Emperor Konoe. As a form of gratitude, Shishiou was forged as an ootachi and was about to be given to him as a prize. But since he’s an old man, Shishiou’s length was cut down to the size of a tachi and was made lighter so that he could use it. Fortunately thanks to Shishiou’s size, his master cannot use him to commit seppuku.
This sword somehow ended up in the possession of Tokugawa Ieyasu, who later handed it over to Toki Yoritsugu, a 'bannerman' in the direct service of the Tokugawa bakufu.
Designated to be an Important Cultural Property at June 22th, 1971.
A speculation came up that he was made by a swordsmith from Yamato province. Other speculates that he might be crafted by Sadahide of Bungo or Sanenari of Bizen.
According to the official tweet, he is a kind fellow with a strong sense of justice. He's conscious of his own height.
The enormous fur that he carried on his shoulder is a Nue, a being that causes misfortune and unhappiness to those around it. It’s a symbolism of the time his master slayed the Nue that was clouding around the Emperor’s castle.