Here's another character. She's technically a good guy, but she's closer to the edge than she realises. The finnicky power is her ability to come back from death. Otherwise she's a magical elven archer with cool light powers. Let me know if there's anything I should change or add! 

[hider=Eltariel]

Name: Eltariel, the Blade of Galadriel)
Appearance: [img]https://66.media.tumblr.com/06b687c5133a20d4937aa5269e8e8d4a/tumblr_orhzwrb46z1tt3wuco1_500.gifv[/img]
Series of origin: Shadow of War
Personality: Eltariel is a quiet woman of reserved but friendy disposition. She believes in the rule of right and tries to solve problems non-violently. Once she decides someone deserves to die though she has no qualms executing them. Eltariel is by all means an authoritarian, and follows the commands of those above her usually to the letter. Though recently she disobeyed Galadriel, and suffered dearly for it, and is now trying to redeem herself of her failure. Still, she can be arrogant and dismissive of others and their thoughts, often believing that her cause and those she serves are the truest, highest, and most righteous authority. She has also been fighting one specific fight for a very long time, and when she is out of her comfort zone she tends to flounder in indecisiveness, or make thoughts based on her base instincts rather than her practiced logic. While she does have somewhat of an ego, she also has very low self-esteem and due to the nature of her abilities and existance places very little value on her own life. 

Abilities: 

Martial Expert: Eltariel is a master of various kinds of weapons, having trained with them and used them for over a thousand years.

Elf: Being an elf, Eltariel does not age. She also has keen hearing and eyesight.

Light of Galadriel: Eltariel wields power granted to her by Galadriel. She can blast out this light in the form of high speed orbs to knock people back and cause them to be inundated with light. She can create kinetic explosions of energy by punching the ground. She can also run at speeds of up to 40 miles an hour once she gets going, and use it to quite literally double jump in the air. It also grants her advanced, supernatural reaction times and environmental awareness, allowing her to percieve the world as if in slow motion. All of these abilities cost focus, and if she uses too many in a row they become taxing on her form and she has to wait to recuperate.

Bow of Lothlórien: Summong an incorporeal bow, Eltariel can loose arrows that travel faster than sound and strike with heat and ferocity. She has ten arrows, and gets more over time or by incinerating evildoers with her light. She can also use two arrows to teleport to a location within one mile of her current location, usually towards her living target.

Undying: Eltariel's life is bound to that of the New Ring, the one she wears on her left index finger. For her, death is literally not an option. There are ancient elven towers in the realm of mordor. Whenever Eltariel is killed, the New Ring will vanish from her body and reappear at the nearest elven tower. There, Eltariel is resurrected back to life on the dawn of the next day. It is an entirely new body, reproducing the weapons and armor she had on her at the time, with all of her memories and experiences intact. Eltariel herself wonders this resurrection is a true resurrection, or if she is just a copy of her soul. In reality it does not matter. Eltariel has died many times on her quest against evil, so the question is moot.

Domination: Eltariel can influence the weak-minded, particularly animals. An enemy must be defeated in order for this to take place, as Eltariel has to place her hand directly on the head of the individual. Those more powerful than her can shrug off the effect entirely, and those under her influence can break free. Mostly she uses it on wildlife to bend them to her will, or on the endless waves of orcs that are attacking Gondor, who care only for violence and can be easily and happily pointed in a different direction. Those who are stern of spirit and mind can shake off her domination. Indeed the person most at risk of succumbing to the New Ring's influence is Eltariel herself, who must continually remind herself of her mission in order to not give into it's inherently evil influence.
Weapons and equipment: 

Light Scale Mail Armor- Lithe and form-fitting, this armor protects her from damage while keeping her mobility unhindered.

Blades of Galadriel- Two magical sword shorts, enchanted to be unbreakable. They are razor sharp, incredibly well balanced, and Eltariel wields them with expert ease. If she wishes to fight nonlethally, she blunts their edges with her spirit energy, making them more into hard hitting stun guns rather than blades. 

The New Ring: The source of her costly immortality. Her right pinky and ring finger have been cut off, the New Ring grants her spiritual, transparent fingers. If the New Ring is removed, Eltariel can be permanenty killed. If there is no elven tower nearby, Eltariel can be permanently killed. As such if there is a risk of the ring being removed she will flee, or kill herself while she still has the Ring on. She can also use the New Ring to meditate and travel back to the nearest elven tower.
Character alignment: Lawful Good
Other: Eltariel is Lawful Good only because of Galadriel's alignment. She is easily tempted by power, though she tries her hardest to do the right thing she has an ego that can blind her to what that truly means. She has been fighting for a long time, and a small part of her wishes it could all just be over. [/hider]

edit: oops, i posted right as you did. Yeah, that sounds cool! Even if he technically isn't a demon, I'd argue he still is by way of being such a huge asshole