First, he came off as a friend to Naruto. Then he was the one that revealed that he was the nine tailed fox. Then he went to attack Naruto, but Iruka jumped in the way. Naruto used his new jutsu, shadow clone, and beat Mizuki for a while.
Later he appears in the series again, and they say that Mizuki was one of Oroachimaru's pawns. He had a curse mark, but it almost killed him when he used it. It was said that he would never be a Shinobi again.