Aiden is an Angel from the Realm of Karid, Siegfried's home world. He was a member of so-called Sanguine Wings, an elite military unit created by the Angelic High Council and charged with extermination of all non-Angel mystical creatures like Demons, Shifters and so on. Or, rather, their mission was to root out such enemies that have come to power among the mortals. They also hunted down renegade Angels who would choose to side with their enemies. Sanguines were known for their brutality and mercilessness, and were feared by all their enemies; even those they were supposedly protecting, mortals, treated them with fear and reverence.
Aiden, a young but promising member of the Sanguine Wings, was a bit of a black sheep among them, though. He did not quite share the fervour of his comrades, and often questioned their methods in his mind; he still followed orders, telling himself that the goal justifies the means. He was also a squire to one of the most zealous members of the organization at that time, Katherine Herzer. He always looked up to his superior and idolized her, but at the same time he somewhat mellowed her own fervour. However, when at the closing of the Great Angel/Demon War, where Angels have pretty much crushed the Demon forces and were poised to wipe out majority of their enemy race, Katherine for some mysterious reasons defected from the Sanguines and sided with her sworn enemy, Demon Prince Siegfried, Aiden's faith in the Sanguine Wings' cause was undermined. Soon after that he himself defected, joining Rin on the other side of the barricades, and following her when Siegfried led his followers out of the Realm of Karid and took them to the Outer Universe.

Aiden © me
 siegfriednox