As this and other threads demonstrates, the church (i.e. Christianity as a whole) does not have a consistent answer as to what it means to blaspheme the Holy Spirit, nor why it is unforgivable. It’s – well, “hotly debated” might e a stretch; let’s just say there are many different ideas but many scholars will admit that we don’t really know what the Gospel author meant here.
As for “who is the Holy Spirit” that could take books to explain. The Spirit is one person of the Trinity, co-equal with God the Father and Jesus the Son. The Nicene Creed says:
We believe in the Holy Spirit, the Giver of Life, who proceeds from the Father and the Son. With the Father and the Son He is worshipped and glorified. He has spoken through the prophets.
The Holy Spirit is usually identified with the breath of God that moved over the formless void before the world was created (Genesis 1). Also with Wisdom (sophia) in other books of the Old Testament. It was the Holy Spirit of God who spoke to the prophets and it is also the HS who works in the hearts and minds of believers to prepare them for God’s Kingdom. Before Jesus ascended, he promised to send a helper and comforter after him - that is the Holy Spirit.