Who is the Holy Spirit?

John 14:16 (AMP)
16 And I will ask the Father, and He will give you another [a]Helper (Comforter, Advocate, Intercessor—Counselor, Strengthener, Standby), to be with you forever—

Jesus promised that he would ask the father to give us the Holy Spirit who would be with nus forever. In fact He said that if we who are evil know how to give good gifts to our children how much more will our heavenly father give us the Holy Spirit if we ask? But who or what is the Holy Spirit?
The Holy Spirit is a divine being, He is omniscient meaning He knows all things according to;

1 Corinthians 2:10-11 (AMP)
10 For God has unveiled them and revealed them to us through the [Holy] Spirit; for the Spirit searches all things [diligently], even [sounding and measuring] the [profound] depths of God [the divine counsels and things far beyond human understanding]. 11 For what person knows the thoughts and motives of a man except the man’s spirit within him? So also no one knows the thoughts of God except the Spirit of God.

God knows everything and the Holy Spirit knows the thoughts of God. The Holy Spirit is also omnipresent meaning He is everywhere at the same time.

Psalm 139:7-10Amplified Bible (AMP)
7 Where can I go from Your Spirit? Or where can I flee from Your presence?
8 If I ascend to heaven, You are there; If I make my bed in Sheol (the nether world, the place of the dead), behold, You are there.
9 If I take the wings of the dawn, If I dwell in the remotest part of the sea,
10 Even there Your hand will lead me, And Your right hand will take hold of me.

The Holy Spirit is eternal meaning He has always been there, He has no beginning and he has no end. Hebrews 9:14 refers to Him as the eternal Spirit.
The Holy Spirit is the Spirit of God, he was there and involved during creation according to the book of Genesis 1 He is the same Spirit that came upon Jesus when he was baptized empowering Him and enabling Him to do all that he did and eventually die on the cross to redeem man.
This is the same Spirit that live is us who have embraced the grace of God through Jesus Christ and surrendered our lives to him.

Romans 8:11 (AMP)
11 And if the Spirit of Him who raised Jesus from the dead lives in you, He who raised Christ Jesus from the dead will also give life to your mortal bodies through His Spirit, who lives in you.

The Holy Spirit is the Spirit of God, He is one with God the Father and God the Son.