He is the Lord of the Crimson Realm to whom Shana is bound. He expresses his will through a divine vessel called the Cocytus, which (in Shana’s case) takes the form of a pendant. He is also a father-like figure to Shana, as he is always aiding her in her in confusion.
Alastor is considered to be the strongest of all Lords or Denizens, even comparable to a god in the Crimson Realm. But even so, he tries to keep the Denizens from consuming Power of Existence. Unlike other lords that have a Flame Haze, his contractor can perform a method called Tenpa Jyousai, which requires a lot of Power of Existence and will manifest him to the world to consume all Denizens around him. This method is his Flame Haze’s last resort.