Sai is of average height, his body is built for speed more so than strength. Lean with muscle from training every day. Silvery gray hair gives the locks an almost blue color look in certain light. At night it simply looks black. Bright blue eyes are testament to his foreign half, though one is much lighter than the other. Usually dressed in white or black kimono's. His armor is black, covering his chest and shoulders when he is preparing for battle. Otherwise, he forgoes the heavy armor, relying on his speed and skill to keep his body from the worst of injuries. A scar marks the left side of his face, his chest, and his arm.
Sai's use of the naginata limits his range of effective fighting. Close encounters cause him to stumble in an attempt to gain distance.
He is without magic, thus going against those who use it give him pause. He's not good at defending against someone who is skilled at the arts of magic.
Holding a secret is just as detrimental as anything. Sai is cautious around his other vassals; perhaps too cautious at times. He trusts them, but not fully and this can cause problems in some situations.
At the end of his naginata comes the demise of many opponents. Sai has mastered his weapon. Strength and speed make him lethal. Sai shows no mercy in battle, and unless ordered otherwise; kills the enemy regardless if they're capable of continuing to fight or not.
Sai has no fear when it comes to a fight. Though he may hesitate when up against a formidable opponent, he goes into the fight with everything he has. Defeat is not an option. Death is just another journey for him.
Sai is half demon, though he keeps that a secret from his companions. Should he ever break the seal binding his demonic half, Sai can wreak havoc and use magic. This is highly unlikely though.
Outwardly, Sai is kind and friendly with his companions. Inwardly, he's constantly fighting a battle of trust and anxiety. Though it doesn't seem like it, he's careful of how he speaks and what he talks about. He throws himself into fights almost too heartily, especially when he's going up against demons. His hatred for demons stems from how he dislikes his half-demon heritage. Though not tempermental, some things can set him off. Talk of demons puts him on edge. Being compared to one can earn his ire and a cold shoulder for several days (or if from an enemy, it will earn them his rage and an all out attack to take their head). Harming his companions can turn into heated anger, including if anyone is being held hostage.
Sai comes from a very powerful family line of samurai. However, the difference lies in that his mother was seduced by a demon and thus he was the result of their copulation. His demon half has always been hidden from those around him ever since his mother's side of the family attempted to kill him when he was ten. The scars are the marks of the demons corruption. In order to hide the truth, the scars have been attributed to a fire when he was just a young baby.
In addition to this, the Yagata family serves the warring Lord Oda Nobunaga. Sai found himself shunned by Nobunaga's vassals. He could not 'prove' himself worthy to serve Nobunaga. His mother, yet again, was his only saving grace. Instead of death, he was sent away with the scars to remind him that he would never be a true samurai.
Sai wandered the country side for a handful of years before he found himself face to face with a man who served Date Masamune. At the time, Sai hadn't known who the samurai was. In the midst of a fight with a demon, Sai encountered the mysterious samurai. Together they purged the night of the monster. Sai was then offered an audience with Masamune. He begrudgingly accepted. Since that night, Sai has wholeheartedly served Masamune and the other vassals. Sai became second in command just a year ago, serving directly under the same samurai who had pulled him into Masamune's home.
Despite the acceptance he's had within Masamune's rule, Sai has never said a word about where he came from, or what the scars really are.