Luke presents Joseph somewhat in the background of his narrative, which is developed from the perspective of Mary. Nevertheless, this Gospel provides a rather definite portrayal of him and of the roles he fulfills.
Joseph is Mary's betrothed, but has no sexual relations with her and is not involved in the conception, which is wholly God's work in her. Being of the house and lineage of David, he is Jesus' legal, genealogical father. It is through him that the Son of God has a genealogy in the human race springing from Adam, inherits Davidic descent, comes to be born in the city of David, and can be called the Davidic Messiah.
Joseph and Mary are never called "husband" and "wife," but are seen as such to form a family together with Jesus. They are called "parents" together, and Joseph is called "father" and referred to as such by Mary. Together they see to the circumcision and naming of the child, obediently fulfill all the prescriptions of the law and temple worship, and are blessed by Simeon. Together they are subject to parental worry and incomprehension while raising Jesus. Together they are obeyed by him. Besides providing for the child, Joseph presumably also instructs him in the law. He is known to all Nazareth as Jesus' father, in a manner that appears totally ordinary.
What is most characteristic of Joseph in Luke's Gospel is his total obedience to God, both in fulfillment of the angel's instructions to Mary and especially in compliance with the law of the Lord. Luke portrays the model of a faithfully observant father who fulfills all the natural and religious obligations of a human parent for the Son of God. He is the obedient man whom Jesus obeys. He lives out this role in the background, a part of Jesus' "hidden life." In faith and in respect for the mystery, Joseph never competes or interferes with the Fatherhood of God. As the time comes for Jesus to begin his mission and publicly proclaim God as his Father, the human father of his childhood quietly fades from the scene.
Taken from http://www.osjoseph.org/stjoseph/scripture/part2sec5.php