Kay, at first, embodied Michael's idealism. When they were first together, he saw her as a part of his master plan to distance himself from the family in every way possible.
Later, after Apollonia died, Michael was constantly justifying himself to Kay, and even lying to her, and perhaps to himself as well about his legitimacy.
"Io sono stanco, sono imbigliato, and I wan't everyone here to know, there ain't gonna be no trouble from me..Don Corleone..Cicc' a port!"
