Known as Ashiya Dōman (芦屋 道満) or Dōma Hōshi (道摩法師), he is the rival of Abe no Seimei who had many magical duels between the two in Japanese legend. One duel had him attempt to trick Abe by putting fifteen oranges inside a box, only for Abe to best him by turning them into rats and saying there were fifteen of them. Douman used a talisman with crossed lines.
"Origin: Japan. An onmyoji from the Heian period of Japan. He was a rival to another onmyoji, Abe no Seimi, user of the pentagram talisman. He is known to have attempted to curse Fujiwara no Michinaga to death. However, he was defeated by Seimi and exiled."
—Shin Megami Tensei A-Mode DDS Dictionary
Douman takes the form of a human and attempts to summon demons from the Expanse by using a Terminal and performing a strange ritual. When the hero and his allies interrupt the evocation, Douman reveals his true form and battles them. After Douman is killed, Pascal/Cerberus leaps at the terminal, only to be teleported away. The heroes try to follow the demon by using the machine, sending them to Shinjuku.
When the Kikyo Amulet is referred to in the A-Mode DDS Visionary, it turns out that Douman had sealed part of his soul a thousand years ago, in order to be reborn once another onmyoji on par with himself and Abe no Seimei appeared. The creation of the Demon Summoning Program seems to have been the trigger for his rebirth, as Douman comments that it is a spell equal to the ones he knew. Douman then states that he plans to build his strength and conquer Earth by extending the Expanse over it.
Devil Summoner: Soul Hackers
"Ashiya Douman, an onmyoji of the Heian era, whose official name is Douma Houshi. He was a rival to another onmyoji, Abe no Seimei. Ashiya Douman used a talisman with crossed lines while Abe no Seimei used a pentagram seal, so these two talismans are often referred to as the "douman seiman"."
—Devil Summoner: Soul Hackers 3DS Compendium
Douman is a demon exclusive to the 3DS remake, available after Nemechi evolves into the snake form, and can be purchased for 100 D-Souls.