He found Seven on a beach 2 years ago and saved his life, but because of this, he's now covered in medical bills that he has to pay off. To pay for them all, he attempts to put Seven into all kinds of odd jobs, but eventually pushes him into an assassinating job.
Dai Bo gives his invention, the GaiBa Egg, to Seven and instructs him on how to become a killer. He also introduces him to Xiao Fei and tells him to use him to flee the scene when he needs it. He soon realizes that Seven is useless as a killer as he always loses, can't bring himself to kill most of his targets, and even saves them.
(Source: Scissor Seven Wiki)
