Seiryu is the third Saint Beast that Yusuke and the gang face. He has the ability to lower air temperatures, which provides him with an arsenal of ice attacks. After the gang defeated Byakko, Seiryu finished him off by freezing him with an Ice Dragon attack and then punching him, smashing the frozen Byakko into pieces.
It was Hiei who took on Seiryu, and defeated him quickly. Seiryu was no match for Hiei's lightning-fast speed, and so Hiei was able to slice him through him eighteen times before anyone could blink an eye.