General Description:
Jack is a simple guy. Not dumb, but simple. He doesn't know a lot about the world, actually he knows hardly anything about the world. He is ill-tempered and quick to anger, and believe me, you don't wanna be too near when he gets angry. He has the attitude that you can reach almost every goal by means of violence. However, he doesn't blindly run into action when he can clearly see that he doesn't stand a chance. In the Shadows he quickly found out that he isn't invincible.
He's quick to judge people and tends to get violent when he feels insulted even the slightest bit.
Oh yea, he doesn't talk much. He rather likes his actions to talk for him.
Also, Jack doesn't have much experience using modern weapons. He is somewhat familiar with shotguns but uses close combat techniques whenever possible. If he has to go for ranged attacks he tends to throw things at his opponents, which he is rather good at. He claims to have killed a guy by placing a pencil in his one eye from 25 feet distance.
He also doesn't have any problem with all the more icky things in life. He eats raw meat, he has no problems dissecting humans (and eating them, too, if it should make sense in the situation), etc. He's an animal, basically. All those human emotional barriers don't really exist for him.

In game terms he is a shamanistic physical adept who is following the path of Fenrir. His specialties are melle combat, throwing weapons, projectile weapons and basically being a really unpleasant guy. His powers are geared towards imitating a wolf (increased strengt and reflexes, improved senses like hearing amplification, high frequeny hearing, low-light vision, scent, etc)

History:
Jack was born and raised in a small village, far away from the civilized world as we know it.
He grew up as one of the few boys in the area, which wasn't exactly a pleasure. He was never able to visit a school since there was not one educational facility closer than 60 miles away from the village and there was no way for him to get there.
His mother owned a little store in the village and the father was a woodworker, living of the local forest which surrounded the village, sawing off trees and making boards and other things out of the wood. Jack always had the air of a lumberjack and this is the reason.
His father also taught him most of the things he knows today. How to handle sharp things like knifes, saws, chainsaws, etc. He also trained his body, so that he could help with the work, chopping wood, lifting logs onto the truck and so on. This work, like pretty much anything in the secluded little village, was done without technology and the little tech they had came from the last century. Some old trucks, wood-working equipment, old machines, mainly run by diesel or gas supplied by a not-so-remote oil-refinery. The only electricity in the village had to be generated by generators, which needed fuel, so electricity was only used when there was no other way. This is why Jack never got comfortable working any kind of machinery more sophisticated than a chainsaw or a truck.
When he was about 14 years old Jack had a dream of a great wolf coming to him and choosing him. This wolf was none less than Fenrir, mightiest of all demons of Norse mythology, bringer of doom, son of Loki (the god of chaos) and brother of Hel (the goddess of the underworld). Jack didn't know why Fenrir chose him or what for and since things like magic and such where thought of as devilish by the strictly catholic local villagers he told nobody about his vision.
Soon he noticed that Fenrir gave him amazing abilities in combat! He started to put his new skills to use in order make people do what he wanted and soon he was feared by almost anyone in the village, even by his parents. His quickly rising anger was infamous and nobody dared to get too close to him. This gave him an attitude of absolute superiority over pretty much everybody. He quickly learned that he could reach pretty much any goal by means of violence.

What then followed is a dark chapter of Jack's life. He never talks about it.
He slowly built up a reign of terror and suppression in the village and as time passed the villagers decided that the situation became unbearable and wanted Jack to leave the village. Of course he wouldn't simply go. The villagers got the few weapons they had and tried to drive him out of the area. There where lots of casualties as Frost fought fiercely. Even his own parents turned against him and, like many others, died. He camped in the woods near the village and hid there, while stealing food from the villagers and killing those that found him. However, a small group of remainders managed to find him and in a final show-down made him flee. Searching for revenge for all the casualties, lost friends and relatives, shattered existences and burnt down houses they want to see him dead and still hunt him down.

He left the village, or what remained of it, when he was 20, with only an old pickup truck, some camping equipment and a few simple weapons in his possession and came to Seattle, without a SIN. In his village people had nothing much to do with the outside world and thus where never assigned SINs.
He noticed that there wasn't much to do without a SIN and therefore turned to the Shadows. After all, this was his place anyways. There he had an outlet for his aggressions, managed to earn some money and generally life well within his own, low standards. Currently he lives in a small hut in the forests just outside of Seattle, heating with wood, eating mainly what he hunts himself.