Michael

Michael is an archangel often brought up by Luke, saying he "reports to Michael directly." This implies he's in a position of authority in the Celestial Realm.

Story
Note: this section may include spoilers

Background
In Lesson 23-5, Barbatos mentions that Michael is the current leader of the Celestial Realm's legion of angels as well as the guardian angel of confectioners.

In Lesson 27-19, it is revealed by Asmo that Micheal used to teach Mammon, but didn't know how to deal with him. This led to Lucifer taking over. In Lesson 23-3, the brothers reveal that Michael had always admired and loved Lucifer.

Main Story
Michael loves sweets of all kinds, according to Luke in Lesson 5-12.

In Lesson 49, Luke has said that Michael often stares at where the demon brother's pictures used to hang when they were angels, claiming that, although he never says anything, Michael misses the demon brothers dearly.