Betelgeuse is based on the character from the same name from the film Beetlejuice, created by Tim Burton and interpreted by Michael Keaton. The character is named for the eighth brightest star visible in the night sky, Betelgeuse, the second brightest star in the constellation of Orion.
In African mythology, this star represents a lion that is keeping its eyes on three zebras (Orion's Belt to Europeans) that the deity Aldebaran failed to kill with his arrow (Orion's Sword to the Europeans), yet did not try to retake his arrow because Betelgeuse was watching the zebras, which made Aldebaran sit there to this day, shivering due to the cold nights and suffering from thirst and hunger.
Betelgeuse first appears in the Factory district, where he has driven out the workers and halted all mining operations. He appears in his giant snake form during the boss fight. After he is defeated, he can be encountered in places such as the Center district as a regular demon in his human form.
Shin Megami Tensei: 20XX
Betelgeuse is the boss of the third floor of the arena. He has no skills, but all his physical attacks inflict exactly 999 damage. The only way to defeat him is reflecting all of his attacks.