Darkjungle was a mercenary that simply appeared one day. No one saw him enter, and the exact date of when it was is shrouded in mystery.
He quickly set about making a name for himself by joining Unknown Science as a demolitions expert. He was human then, however, over the years he slowly became more and more robotic.
Then Season 4 Episode 1 happened...
Whilst repenting over his life with a bottle of Jack, he faced the red mist. Doing the only thing he could, he charged in. He was stripped of his flesh and become almost fully synthetic. The only thing he had left of his human days was the brain in his metal skull.