The Holy Spirit is God finally reaching us to get inside of us.
The Holy Spirit is the Father and the Son coming to man to impart, dispense the divine into man's being.
The Holy Spirit is:
1.) Holy - meaning He is separated from all things common. Everything that exists is in one catagory of "common" things. And God Himself is in a whole other catagory. God is unique in an ultimate sense. He is different from all other things which have being. God alone is holy.
So this essence of God is unique and uncommon; separated from all other things which exists. This is "holy."
One caveat: In the Bible things which are consecrated to God are also called "holy", ie. holy angels, holy anointing oil.
2.) Spirit - is a word meaning wind or breath. This is a limited way in which human language can discribe something abstract in a divine and mystical way.
Holy Spirit is God Himself. And Holy Spirit is
"pneuma" or "breath". And this pneuma is Jesus Christ Himself. The Holy Spirit is Jesus Christ in another form. The Holy Spirit is the man Jesus Christ in His
pneumatic form.
The Holy Spirit is God in Christ put in a form in which He can enter into man's innermost spiritual being.