Saint Michael the Archangel isn’t a saint in the traditional sense, but rather he is an angel and the leader of all angels and of the army of God. This is what the title “Archangel” means, that he is above all the others in rank.
However, Michael the Archangel is still called a saint because he resides in heaven in communion with God. Michael the Archangel has four main responsibilities or offices, as we know from scripture and Christian tradition.
The first is to combat Satan. The second is to escort the faithful to heaven at their hour of death. The third is to be a champion of all Christians, and the Church itself. And the fourth is to call men from life on Earth to their heavenly judgment. Very little is known about Michael the Archangel other than what we know from scriptures, which themselves are sparse. In Daniel, he is mentioned twice.
The first time as one who helped Daniel, and the second time he is mentioned with regard to the end times of the world, a time when he will stand and defend the righteous. His next mention comes in the Epistle of St. Jude, where he is said to guard the tombs of Moses and Eve and has contended with Satan over the body of Moses.
The final mention is in Revelation, where St. Michael and his angels, do battle with the dragon.
There are other scriptures where St. Michael is implied, but not mentioned by name, such as the angel; who defends the gate to Paradise.
Today, St. Michel the Archangel is invoked for protection, especially from lethal enemies. He is also the patron of soldiers, police, and doctors.