Ka-gyou can be summoned by typing his name in the Computer Room Thi-eng, in the Fire-Tower of Time Tong-tah, in Shi-chieng.
When the player is Kai, Ka-gyou gives them the mission to extinguish Kai's candle in the Plains of Chi-u.
Spoiler warning! This article contains important plot details.
At the end of the game, Tou-gyou reveals that with the other three kings, Ka-gyou overthrew him, cut him up into eight pieces, and scattered the pieces throughout the land after they found out about the use of human souls.