>>531820079
Halos seem to be a sign of divinity. Students have one because they are supposedly forgotten gods. Communio Sanctorum(Hiero and Greg) bosses and Deca Prophets are treated like or related to godly beings in the story so they also got halos unlike Slumpia bosses, Perorodzilla and Kurokage. Beatrice's true monster form having a halo indicates that she was a divine being or at least acquired divinity through her rituals. Kivotos itself might be considered as a divine region/being and that is why the sky has several halos.