The Holy Spirit is the third side of the trinity. In Catholic religions the trinity is 'three in one'. We believe in one God but in three forms. God is the Father. Jesus is the Savior. The Holy Spirit is the Bearer of Courage. Here is the Apostles Creed ...
I believe in God, the Father Almighty,
Creator of Heaven and Earth
And in Jesus Christ
His only Son, Our Lord
Who was conceived of the Holy Spirit
Born of the Virgin Mary
Suffered under Pontius Pilate
Was crucified, died and was buried.
On the third day, he rose again
He ascended into Heaven
and is seated at the right hand
of God, the Father Almighty.
He will come again to judge the living and the dead.
I believe in the Holy Spirit,
the Holy Catholic Church,
the Communion of Saints,
the forgiveness of sins,
the resurrection of the body,
and life everlasting.
The Holy Spirit is also referred to as the 'giver of life' and the spirit of truth. It is the Holy Spirit that appears during baptism. It is the Holy Spirit that is the forgiver of sins during the act of contrition. The Holy Spirit is associated with knowledge, and more importantly, wisdom.
The big day for the Holy Spirit is Pentecost which happens after Easter and after the Assension into heaven of Jesus Christ. Pentecost is when the Holy Spirit decended and gave the gift of tongues to the Apostles so they could go and preach God's word to all people and create the Catholic religion.
Every year teenaged Catholics (and older) become 'soldiers for Christ' by taking the sacrament of Confirmation. This is where they are again filled with the Holy Spirit and are strengthened to become adult members of the church.
Actually, the Holy Spirit is very important because it is the Holy Spirit that bestows strength so we can embrace a full religious life. I've read that the sins against the Holy Spirit (sins of obstinance, despair and presumption) are considered unpardonable.
Act of Consecration to the Holy Spirit
O Holy Spirit,
Divine Spirit of light and love,
I consecrate to Thee my understanding,
heart, and will,
my whole being for time and eternity.
May my understanding be always submissive
to Thy heavenly inspirations,
and to the teaching of the Catholic Church,
of which Thou are the infallible guide;
may my heart be ever inflamed
with love of God and of my neighbor;
may my will
be ever conformed to the divine will;
and may my whole life
be a faithful imitation of the life
and virtues of our Lord and Saviour Jesus Christ,
to whom with the Father and Thee
be honour and glory forever. Amen.