Shuten-douji is an Oni, often called the leader of all oni in the mountains, for he is the most infamous and the strongest. Two stories exist in regards to his origin. One story says that he was a handsome young man who shook the hearts of many women, but he was cold and arrogant to all of them. So, one of the girls who had her heart broken by him sent a letter imbued with a curse. Upon reading that letter, the letter burned and clung to Shuten's face, turning him into an Oni.
Another story says that he is in fact, a son of the Yamata-no-Orochi (which would explain how he could drink large pots of alcohol at once). He was sent to a Buddhist temple to control his demonic blood, but when he wore an oni mask at a festival, the mask merged with his face and turned him into an oni. He has several Oni under his command, the most famous being Ibaraki-douji, an oni who is either said to be Shuten's son, right-hand man or even his wife.
He is grouped with other infamous youkaiTamamo-no-mae, Nurarihyon and Sutoku Tenno as the most dangerous youkai in Japan.
"Origin: Japan. An oni appearing during the Heian Period. He kidnapped people at Mt. Ooe and devoured them. He was later made drunk through a plan by Minamoto no Yorimitsu and decapitated. However, his head is still capable of attacking on its own."
—Shin Megami Tensei A-Mode DDS dictionary
DemiKids Light & Dark
"A violent and extremely dangerous demon that enjoys a good party."
Shuten-doji was a general of the Imperium and appeared alongside the other generals at the beginning of the game. However, he betrays them on Jin's route, where he decides not to fight while his Imperium troops continue in the fourth stage of Wind Makai. Upon defeating all his soldiers, he joins the party.