Wait. Do we know what he did before he was 001?
Plane Crash >
Suffered Amnesia and trained in the Numbers camp until he's become 001 >
Memory came back, because of that he couldn't follow camp orders anymore (killing children, woman etc. without questioning)>
After regaining memory he left the camp, rendering numbers following him unable to move, but someone else killed them (Cause for numbers hatred towards him, since they were made to believe he killed them)>
He's become a mercenary, until he rescued Major Kang in a mission, who in turn found his grandpa and sister>
Ijin returns and we end where we're at now