Spenta Mainyu is the Holy Spirit of Ahura Mazda. In Zoroaster's initial vision the first act which he conceived Ahura Mazda performing was the evocation of the six lesser divinities through his Holy Spirit, Spenta Mainyu.
It has been theorized by some scholars, particularly Maneckji Dhalla, that Ahura Mazda was hypothetically the father of the twin Spirits (Y 30.3), Spenta Mainyu and Angra Mainya. However, when assuming that this is true one abandons the absolute doctrine of the separation of good and evil. A.G.H.
Boyce, Mary, Zoroastrians: Their Religious Beliefs and Practices, New York, Routledge, 2002, pp. 21, 213