Hebrews 1:2 (New International Version)
2but in these last days he has spoken to us by his Son, whom he appointed heir of all things, and through whom he made the universe.
I believe that the author is specifically speaking about Jesus here, and referencing how He (Jesus), who the Lord has appointed as heir of everything, along with God the Father and the Holy Spirit...and Jesus (the Son) will be the one who comes back in the last days.