Sai Kagerou is a sword master and member of the armed division of Yami.
Sai Kagerou is a powerful and tall swordsman of the Yami's Armed Division. Long time ago (when Shigure was a little kid), he visited the house of Hachirobe Kosaka (Shigure's father) to receive Setsunamaru, a especial sword forged by Hachirobe himself. He later left the house but he couldn't wait to use his new sword so he killed a bird just for fun, the problem was that this bird was a important friend of Shigure, so she tried to attack Kagerou (at the same time Hachirobe went out of his house and told the swordsman to go away). Kagerou dodged Shigure's attack and when she tread on the ground he was already in front of her telling her that she needed a few years to challenge him.
Kagerou is a temperamental man and uses to cut people around him for several reasons, for example: if someone stands behind him, Kagerou cuts that person; if someone calls him using terms like "huge", "disgusting" or any insulting word, Kagerou cuts that person; if someone gets too close to him, Kagerou cuts that person, etc. Kagerou doesn't like women and usually doesn't recognize their faces, but on the other hand he treats and love the swords like women. Setsunamaru was his most prized sword, but after he lost against Shigure Kosaka who took Setsunamaru away from him, Kagerou got depressed and lost confidence.
Time later Mikumo Kushinada and her disciple Chikage Kushinada visited him in a dilapidated house full of cuts and broken swords which didn't resist his "technique".
Mayor Story Arcs
Journey with Shigure
After Kenichi almost died in his battle with Kokin, he accompanied Shigure in her little journey to recover a sword that was forged by her father, the purpose of this was to help Kenichi to get rid of his fear in order he could continue with his martial arts' life. This journey was a trap, so a lot of warriors met in the same place to kill the famous Kosaka Shigure, among these warriors was Sai Kagerou, who (after cutting unconsciously some of his allies) waited seated until Shigure and Kenichi defeated all the other warriors. When the battle was over, Kagerou and Shigure started a little conversation where is revealed that he was the one who sent Shigure a picture of his sword Setsunamaru in order to make her come to his place. Kagerou also tried to had Shigure read his guide but she cut it into pieces and asked him if he remember her, Kagerou stared at Shigure but he didn't recognized her and got angry emiting an strong ki.
Kenichi was impressed and also could see Kagerou's huge Seikuken which cointained both Shigure and him.