Name: Cain, Lesser Envy
Age: Thousands of years old, at this point he’s lost track
Race: He likes to think of himself as the human he originally was
Appearance: 
Cain is a 5’8 middle eastern man with a somewhat muscular build who looks to be in his 30’s, with brown eyes and black hair. He keeps his curly hair short and has a well maintained beard covering his jawline. He bears the
Mark of Cain on his right forearm. He often wears well tailored suits with many more inside pockets than would be standard.
Personality: When Cain was younger he had a lot of emotional issues he had to work through. He wanted back the life he had before he was exiled from paradise. Even after building the first city and ruling over his own people, he wanted more. He would often go out and simply take things that he wanted from anyone he came across.
After thousands of years though Cain has mellowed out in that regard. He still sometimes takes a physical item that he thinks would be better in his hands than in the hands of someone else. But now he longs for things he can’t simply take from others. A sense of belonging, a sense of purpose. He is envious of those who have these things, and they are things he fears he will never have.
Background: Cain did indeed kill his brother all those years ago, he won’t deny it. He did it for the reasons you think, he wanted what Abel had. Abel was the favourite, and being young and stupid, Cain thought if Abel was gone then
he would become the favourite. He was wrong, and for his crime he was exiled from paradise and cursed with signature mark. The part about it reflecting harm back sevenfold is a lie, it was simply made to grant him immortality. He would never die of old age. Things are never quite that simple though, the curse also imbued him with strange abilities.
Ages passed, Cain saw the rise and fall of civilizations. He found success for himself wherever he went, trying to sate the envy that started this long life he has now cursed to. His envy even extended to children, though not in the same way it did to others. He always envied their innocence, their limitless optimism that he could never again achieve.
When these feelings get to be too much, Cain will use the one magic spell that he can truly call his own, that he did not copy from anyone else. It is a spell that allows him to view the memories of a recently deceased person, to experience the innocence of childhood and the normal life that he will never have again. He will go out and murder someone society won't miss, it's easier to keep his hands clean that way. The spell stops the soul from going to the afterlife, instead subsuming it into himself. He feels no regret for these souls, they were the dregs of society anyways, and they had something that he wanted. His feelings are never sated for long though, and he repeats the process, trying to fill the void inside himself.
The spell gives Cain any abilities the deceased may have had, in exchange he sometimes hears their disembodied voices in his head. On very rare occasions a particularly strong personality can cause Cain to say what they're thinking. He has learned to keep them in check and mostly tune them out at this point.
Nowadays Cain is on the board of directors for several companies and lives off of thousands of years of wealth accruement. The Detective somehow got his private number which was enough to get Cain intrigued. Hearing the job was to protect children, well, Cain would never be able to find it in his heart to say no.
Skills and Abilities: Cain is skilled in hand to hand combat, but his real strength comes from his cursed mark. On top of keeping him alive and looking young, it has given him the ability to copy the traits of objects. This is often used to copy stone and iron, making his body tough to injure and incredibly strong. He keeps uncommon items in his pockets to be prepared for circumstanced where specific properties are very useful. Such as tungsten to make himself basically heatproof.
He can also copy the abilities of others, this includes both mundane skills and supernatural ones. His level of proficiency will change based on how able he is to do the skill normally and how often he has copied this particular skill.
For a mundane example, copying an expert singer would allow him to sing well, but he could not hit notes his vocal chords can't make nor would he have an increased lung capacity. If he copied a gymnast he would have better balance and maneuverability but it would not allow him to do the splits because his body is not that flexible.
In terms of magic, it would be as though he was a novice of whatever type of magic his target could do. Of course, being around for thousands of years has given him an understanding of most magic systems. He can use basic spells of almost any type he copies, and if he copies the same magic multiple times he will get more proficient.
Weakness: The most mundane of Cains weaknesses is that he can still be killed with a simple gun or knife to the gut. If taken by surprise then his long life would end. Cain also has a deep fear of wolves, and facing them would greatly impact his ability to act rationally.
On the magical side of things, gaining the properties of things also includes any obvious downsides. Such as stone and metal causing him to move slowly. He also can’t maintain this change in body permanently. The amount of time he can maintain a property changes based on how much stress he is under. The more activity he’s doing, the less it will last. When copying the magical ability of someone he is not as proficient as they would be, especially when it comes to magic. He would need to practice after taking their ability to become more proficient.
Finally, his magical abilities need physical contact to work. He can not copy someones abilities without touching them (through clothes works). The longer he is in contact the more he can absorb.
Equipment: He carries a knife with him, the Cursed Blade. It is carved from the rock he used to kill his brother. It will always magically materialize in its holster at his side after a short time if it is separated from him. As far as he knows it can kill anything as long as it is wielded by him.
Quips: Cain doesn’t like dogs, he’s definitely a cat person.
There isn't a day that goes by where Cain doesn't think about his brother, and the regret he has for killing him.
The story of the Mark of Cain reflecting damage was propaganda made by Cain himself, he thought it could get him out of a tight situation if need be
Cain has a working relationship with Aodh. He came upon her in a cave, or more specifically he came upon her sword. He had always admired the swords of the angels, they were much nicer than his blade. So he tried to take it. Aodh came out of the cave and quite nearly ripped his arm off before seeing his Mark. Luckily she decided to strike up a conversation instead of striking him down on the spot. Being two souls who were both banished from paradise gave them a lot in common. They went their separate ways, but every few centuries Aodh would call up Cain asking for his help. He never said no, her targets were always ripe for acquiring memories for him to absorb, and perhaps they had some nice worldly possessions as well.
