Seiryū is a member of the Su Shen. He specializes in learning everything about his opponent's technique after seeing it once, allowing him to take advantage of any weaknesses that exist therein. This comes in handy against Saitō Hajime's Gatotsu, allowing Seiryū to get the advantage over him when they exchange blades the next two times. Unfortunately for Seiryū, however, Saitō is able to adjust the attack as he pleases and in the next exchange, he shifts the direction of his attack slightly to the right, allowing him to break through Seiryu's guard and defeat him.
Kenshin notes that if Saitō's Gatotsu was his only advantage in battle, he would've been able to defeat him in their first encounter during the Bakumatsu. He goes on saying that Saitō's true strength are the three words of the Shinsengumi slogan that are deeply embedded into his body and soul - Aku Soku Zan, meaning slay evil immediately.