At first reading this would be a reference to God himself, the first person of the Trinity. That is confirmed by the opening words of the book of Revelation. In Revelation 1:1 we read of several links in a chain, where the book of Revelation originates with God, then is passed along to Jesus Christ, then to the angel, then to John, and ultimately to the seven churches. The opening words of the epilogue recall this chain and draw attention to its beginning.
Of the five links in Revelation 1:1, Revelation 21:6 mentions three, that is, the first (God), the third (his angel), and the last (his servants). The second link (Jesus Christ) and the fourth (John himself) are missing. That is because the emphasis now is not on the links of the chain but on the absolute reliability of these words,
which formed the content of the revelation—and are the reason his servants
are obligated to respond as they must. These are God’s words.
6 And he said to me, “These words are trustworthy and true. And the Lord, the God of the spirits of the prophets, has sent his angel to show his servants what must soon take place.”