The Holy Spirit.
The Holy Spirit is the third person of the Trinity, along with God the Father and God the Son. He is the comforter or paraclete promised by Jesus in the Upper Room Discourse (John 16:7) and was poured out on the church on the day of Pentecost (Acts 2). Hence, right understanding of the Holy Scripture means that we recognize the Holy Spirit as the fulfillment of Jesus' promise in John 16, and that the comforter is not a later prophet or other holy man. The Holy Spirit is also God at work in the hearts and souls of His human creatures, creating faith in them (I Corinthians 12:3).
Note that the Holy Spirit, like the Father and Son, is a person--a being acting according with purposes and capable of rational interaction with others. He is not merely a divine force or influence, for Peter condemns Ananias and Sapphira of lying to him (Ac. 5:4,9). One cannot lie to gravity or electricity.
The Holy Spirit is God as companion to all Christians. He is the one who polices and changes their minds, opening them to truth, and informing them, through the Scriptures, of God's precepts.
The dove is used as a symbol for the Holy Spirit in much art.