That is clearly a reference to John himself. As the man commissioned to write down Jesus’ revelation for the sake of the churches, John has a role parallel to that of the angel, that is, both the angel and John are messengers of the gospel. The word “angel” actually means “messenger.”
10 Then I fell down at his feet to worship him, but he said to me, “You must not do that! I am a fellow servant with you and your brothers who hold to the testimony of Jesus. Worship God.” For the testimony of Jesus is the spirit of prophecy.