The Holy Spirit is the third person of the Holy Trinity. He reigns with Our Father and Jesus Christ. This belief was expressed from the earliest days of the Church in the Apostles Creed. It professes faith in the Holy Trinity — Father, Son (Jesus) and the Holy Spirit
.… I believe in the Holy Spirit, the holy Catholic Church, the communion of saints, …
— Apostles Creed
For more information on the Holy Spirit please see this page.