Sebastian

 In the 'secret force' investigating a particular Incident that Deimos was involved in and is in the act of avoiding. After some investigation, eventually, he comes across Deimos and tries to gently ease him into turning himself in, saying it'd look better for him and what not, but they both know how bad it already looks for him. However, if Deimos doesn't come willingly, he has plenty of other plans to bring him in by force if necessary...

After some drama, he'd eventually become a friend to Deimos and, potentially, Mark by correlation. He'd get to the point where he'd 'accidentally' let Deimos get away, or get a phony 'lead' in the investigation, throwing months of work away with the false leads. His change of heart would come after understanding the circumstances/truth behind the Incident after Deimos proves himself against Sebastian's beliefs against him.

A more light hearted type of guard, he comes off as rather friendly and approachable, making investigations easier with people liking him enough to talk freely and let little tidbits of information flow freely, whether they meant to or not. Despite his friendly appearance, he's rather calculated and, if the need arose, would easily show off his more menacing personality. Though he generally means well, there have been plenty of times where even he grew tired of it all and lost his patience to deal with things 'properly'. ((aka he's got a lil bit of a temper if something's been troubling him enough))

No one speaks up about any such incidents as, in reality, they do make results... Even if not all of the new recruits agree with the methods.

Once upon a time, he did have a younger brother.

In his teens, his home was ransacked by a rogue mage, his brother having been the only one home. The incident left him bitter towards rogues, leading to his enlistment into the organization. As a result of his experience, he has little patience for rogues, very seldomly letting them even get a word out before he sets upon them.

The only reason he had tried to convince Deimos to turn himself in is because of Deimos's reputation and the severity of his crime, add on the fact of how long he'd eluded them, Sebastian had thought him to be a far greater challenge than he ended up being. It is only after much strife and confrontation that Sebastian finds himself listening to the truth behind the misconstrued facts of Deimos's case, the files having painted him a vicious, heartless man with no care for life. 

After the realization of how misinformed the organization was on the matter, he starts to reevaluate his allegiance to the organization, wondering just how many cases were misunderstood in a similar way.

His magic skills focus on fire.

extras:
-Sebastian often finds himself dreaming of living a happy life along with his brother. (idea courtesy of @DEADNOIZ though I changed some of it)
-He can get stuck in his ways and has a hard time believing something different from what he'd previously believed (aka Dei's whole situation)

personality traits:  Bull headed, Quick to jump to conclusions, Unforgiving, Relentless, A bit of a cruel streak...
things he hates:  Rogue Mages, Liars, People who’ve hurt their family, Betrayal, The seeds inside of raspberries
things he loves: His bandanna, A small picture he’d had taken/painted with his brother shortly before he died, Raspberries, Crepes, His brother
Other characters he gets along with: Sten, Nima
Things he will always remember: Finding his brother’s body, Finding the man who killed his brother, The number of rogue mages he has killed, The more nightmarish stories of criminals he’s tracking down
physical things about him: His bandanna (from his brother), Cowlick, Hair color: Blond, Eye color: green, Build: Decently muscular but not too much
Things he couldn’t live without: He’d thought his brother, The picture of him and his brother, His bandanna
Things he wishes he had: His brother back, The Truth about Everything

((here's his playlist))

if drawn, you do not need to include the arm band, I plan to take it out of the design. At the same time, do not worry if you draw the arm band, it is not worth the worry.

Changes in his design are coming