All of your posts have been about earliest Japanese history so I'll assume we're talking about that time.
Basically, the role of the emperor did not change that much in early Japan. When the Chinese influence started to be felt, there was a move to try to create a centralized government with a trong emperor like China had. However, this did not really work and Japan moved back towards feudalism. Imperial power was at its height when Nara was the capital in the 700s. By the 850s, the Fujiwara were running Japan with the emperor as a figurehead.