Who is the Holy Spirit?
“For who among men knows the concerns of a man except the spirit of the man that is in him? In the same way, no one knows the concerns of God except the Spirit of God. Now we have not received the spirit of the world, but the Spirit who is from God, in order to know what has been freely given to us by God.”
— 1st Corinthians 1:11-12
He is the Spirit of God and is fully God.
(Taken from “Know the Truth:60 Questions and Answers on Christian Beliefs” from First Baptist Church in Tallassee, Alabama; Based on the 2000 Baptist Faith & Message)