Or so says St Robert Bellarmine:

There are therefore in each realm two kings: one who is visible, a man; another who is invisible, an angel.  And in each church there are two bishops: one who is visible, a man; another who is invisible, an angel.  And in the universal Catholic Church, there are two supreme pontiffs established under Christ: one who is visible, a man; another who is invisible, an angel.  We believe that this angel is St Michael. For as once the synagogue of the Jews venerated him as their patron, so now does the Church of Christians (‘The Ladder of Ascent to God’, 9th step.)