Deadpool (Wade Wilson)

Deadpool is a former SHEILD trainee, but became a mercanary, and earned the title "Merc with a Mouth".

Origins
According to Deadpool, his mother was captured by ninjas 30 seconds after he was born. Also, Spider-Man was there because of "genetic engineering or whatever". He also claims to have gotten his powers from falling into a radioactive swimming pool as a teenager. In reality, he was just some kid picked on and beat up by a gang of thugs until Nick Fury recruited Deadpool into his Young Heroes program. There Deadpool trained alongside Nova, White Tiger, Iron Fist and Power Man, but eventually became disillusioned with Fury's goals and wanted to get revenge on the people who tormented him all those years. He left the program to become a "freelance hero" shortly before Spider-Man was brought in.

Merc with a Mouth
Deadpool later teamed up with Spider-Man to track down "Agent McGuffin", a rogue S.H.I.E.L.D. agent suspected of selling S.H.I.E.L.D. secrets to Taskmaster. After the two took down Taskmaster, it is revealed that there is no Agent McGuffin, and Taskmaster stole the secrets off Deadpool. Spider-Man rejects Deadpool's offer to split the money in exchange for keeping his secret, and the two fight. Spider-Man eventually gets the best of Deadpool, and Deadpool reveals to his better how he became the person he is. Spider-Man, understanding how easily he could have been just like him, allowed Deadpool to get away.