Michael is the first and oldest of the four Archangels created by God, making him the very first angel ever created, as well as one of the oldest and most powerful beings in creation in the universe. During the War In Heaven in the distant past, Michael banished Lucifer from Heaven on God's command, later restricting him to a Cage bound by 66 Seals. He was the Viceroy of Heaven in God's absence, and commanded the Heavenly Host before he fell into the Cage with Lucifer. As the highest-ranking Archangel, he issued the commands, which went to the higher-ranking angels to disperse the orders down across the angelic chain.
Michael is a rather complicated being, having many sides of his personality. He is a strong but strict leader, having set up a rigid and sturdy hierarchy in Heaven, along with strict rules and harsh punishments; likewise, his rule was described as by Castiel as "corrupt," although it wasn't exactly oppressive. Despite this, he kept his brethren in the dark on many important matters, including God's leaving, and punished any who displayed disobedience with death.
To his father God, however, Michael is fanatically loyal and blindly follows God's orders without question, as sees himself as a "good son", and that is what matters most to him. He loves his father dearly and, despite having not seen him in thousands of years, never lost his faith or his love for him. Due to his zealous trust in his father's commands, Michael doesn't believe in free will, not even for himself, believing everything that occurs is how his father planned it.
Michael also seems to think of himself as magnanimous when he promises to leave Dean intact after having him as a vessel and erasing the memories of his parents of Anna's attack. He was also shown to be a relatively forgiving person, as he forgave both Uriel and Zachariah, even after Zachariah's constant failures, implying that, despite being strict or even cruel with them, he does not enjoy killing his own kind. Michael could be wrathful, particularly with anyone who interferes with his destiny.
When it comes to humans, Michael seems to be somewhat uncaring; he does not care if or how many he kills. Despite this, Michael also doesn't seem to hold any true hatred for humans, never specifically going out of his way to harm any, or even really bothering about them before the Apocalypse occurred. Unlike Lucifer, he was not angered by God's love for humans being greater than for him, and when asked to bow before them by God, Michael bowed.
Michael is in fact in many ways a tragic figure, as both the people he loved the most either betrayed or abandoned him, and all he ever really wanted to do was to prove to his father, God, that he was a good son, the one thing that he never actually managed to achieve.