Sorry if I look messy when describing this character, but he isn't set in stone in my mind and I often add some changes based on your feedback, generally when I find a concept cool and wants to add it. The nordic look also came into my mind after a session on For Honor playing the Highander, that's why most inspiration come from him now.
I will clarify some part on Aaron. He is indeed a former knight of the OOTS, but the parallel with Nero is that he was also a orphan. This background could also be used to solidify this hatred for demons in general, like having one of his parents being a sorcerer and eventually causing the destruction of the whole family by messing with demons. A variant of this would be that he was "gifted" with his strange power when young, and this attracted demons who then slaughtered his family and he survived using his power. This power would make him seen as a bad omen, causing people to shoo him away (another parallel with Nero and his arm).
Either way, he is an orphan who was taken to the Order after being found in Fortuna, and showed a great talent for combat, leading him to be trained by Credo. Having no family unlike Nero who grew with Kyrie and Credo, he only had the Order in his life and took his duty very seriously (unlike Nero). He would hide his power as much as he can, but his reputation as a bad omen would make him a loner in the superstitious city of Fortuna and even among the Order. This would push him to improve his skill to be accepted for his strength in battle, but still no one would dare to link with him.
Finally, in a final attempt to find someone, he would turn to Kyrie, a person he always found beautiful and hopeful. He would then be rejected by Kyrie, who had already feeling for Nero, and I will modify my take on his by making Kyrie try to reach for Aaron only to have Credo stop it and order Aaron to stay away from her, again because of his reputation as a bad omen.
This would be the final push for Aaron. Unable to find anyone to bond with, he would turn on the last thing he has : the legend of Sparda. Interpreting Sparda as a demon who find his place by growing strong enough to be able to protect everyone, he would embrace his magic in order to gain power. But his fanatical view on Sparda coupled with a dried out patience for other would turn him into a cold and calculating fighter. Him openly using his magic would also push him even further from the knights of the Order, his presence only tolerated because of his ruthless efficiency. Seeing Nero with Kyrie would also make him extremely jealous, leading to a hatred for Nero. But he would never unleash his hatred, still believing that he can be loved by becoming a protector for everyone just like Sparda did. Until one day when he would finally snap.
This would occurs during a fight with though demons alongside others knights of the Order, including Nero. During the fight, Aaron would find himself in a tight spot, and to get out of it, he would use others knights as bait in order to distract demons long enough for him to recover and deal the final blow. This would not go unnoticed, and others knights would lash out at Aaron for his actions. He would ignore it as he usually does until one knight push him, causing his hatred for the people who never accepted him to rise. He would then start attacking other knights, killing some in the process until Nero intervene. After a brief fight, Aaron would be defeated by Nero demonic strength, subdued, then banished from the Order by Credo. But Agnus would then step in to take advantage of the situation. After discussing with a chained Aaron, Agnus would manipulate him into joining his research, claiming that most knights, including Credo, are ignorant of anything about demons and that he hold a huge knowledge about demons. This would attract Aaron's attention, who would accept the deal, finally finding someone not afraid of him.
His relationship with Agnus would be mostly professional. Agnus would ask for demons, Aaron would bring them in, and then Agnus would share some knowledge with him. This would be something new for Aaron, and he would enjoy Agnus compagny. As for Agnus, he would develop a huge respect for Aaron because of his skill and will to learn alongside his logical mind. This moment could be an opportunity to give another shot at Agnus, for he was sadly a victim of DMC 4 story being torn apart. Plus his death scene is so weird. He look like he didn't took the bullet and just fainted. Could be a potential come back for him with Aaron saving his associate as a parting gift by removing the bullet from existence. With Nico being his daughter, Agnus could have a good synergy with her, or even serve as Aaron sidekick and help developing his weapon and runes.
Back to Aaron now. The negative view of Agnus on humans would have his effect on Aaron, making him dislike common people on a certain extent. But he would still follow his ideal of being a protector of humanity, believing them to be mostly misguided and in need of a good leader. This would also be why he didn't took part to the Ascension ceremony, with Agnus respecting his wish. This is also during his time with Agnus that he started developing his runic magic.
During the attack on Fortuna, Aaron would have watched from afar, mostly observing Dante in action. But witnessing Sanctus use the faith of the people of Fortuna would reinforce his idea of the need for a strong leadership for mankind and not someone obsessed with power. The fall of Sanctus would comfort him in his idea of not using demonic power, and he would then leave after saving Agnus, whom he consider quite similar to him (rejected by others, logical mind, sharing the idea of leadership for mankind even tho he choose another path).
After the fall, Aaron would become a demon hunter in order to live his life. Nothing special would happen until the end of DMC 5 and the realization of the existence of demons by the public. With this, more people would try to master demonic magics, cults would appear and Aaron would add them to his hunting list. Witnessing his talent and convictions, some people would ask to join him in. He would accept and form his own group of demon hunter, knowing well that working with others expand your possibility. At some point, he would meet Agnus once more, and take him in his group. The two would become close friends, and Agnus would expand on the runic magic again, perfecting it. All of this until Aaron stumble into Nero once again during a mission, tracks him down to his house and meet with Kyrie again. This would greatly disturb Aaron, who distanced himself from everything long ago, and now having personal feeling to deal with about Nero and Kyrie.
And that's it. I added Agnus as Aaron sidekick because I always liked him and though that he didn't got a chance. Also with what Dante told him before shooting, it would be interesting to see how it has affected Agnus.
I also expanded on his backstory in the Order to make his view on the world more fleshed out and understandable. The goal is to make him opposed to Nero by having him think more logically, being more precise and efficient, and devoid of empathy for other. But despite his lack of empathy, Aaron would still follow this own ideal on Sparda, making him more complex as a character than just another Vergil. Finally, he would be a push for Kyrie to become a true character, to push Nero into a more personal struggle and to make justice to Agnus.
