Demon: Raphael
Requirements: Uriel (Any variation)
Summon Uriel (any variation), then talk to Knight Templar Amami in front of the Arcadia entrance, who will give you a quest to kill the Mad Seraph Raphael, and will specifically request you to deliver the last blow together with Uriel. With that done, head to the Arcadian guard again and talk to him with Uriel summoned. He'll give you the Raphael Plugin: The Blessed Light in exchange for your services.