Bob Singleton is a former agent with the CIA.
To be accurate, he was an assassin for the Agency, killing who he was directed to when he was told. He did it for far too long, in his opinion, and he was very good at it. If Langley needed a target removed and the chances for success were low, Singleton was the man they thought of because they knew he knew how to survive. That was then.
Now, when we meet him in the first recorded adventure, those days are far in the rear view mirror. Now he lives in a cardboard box covered in plastic to help it survive the frequent runoff from the eaves of the building he uses as a support. He wakes each morning hoping there is something left in whatever bottle he took to 'bed' with him the night before. He doesn't worry too much about food because the mission just down the road a bit will provide the sustenance he needs. It has been doing so for several years now. He walks a lot, partly for the exercise and partly because he has no other transportation. He owns no car and no taxi would pick him up and he knows he smells horribly so the bus is something he avoids. All things considered, he is surprised and a little pleased that he is still in "okay shape".
Back in the days where it seemed to matter to him, the doctors labeled his troubles as PTSD. He now just knows it as memories of a life spent doing very unpleasant things to very unpleasant people. The dreams made sleeping difficult without the aid of alcohol and alcohol made doing anything else difficult.
We are told that after several years on the street, he "was an old hand at avoiding trouble" and for the most part that is true. It isn't, though, when we run into him at a local mom-and-pop grocery mart planning to steal a bottle of vanilla extract because of its alcohol content but he is interrupted when a trio of young hoods try to rob the place and plan on killing the teenager behind the register and as bad as Singleton was feeling, he just couldn't let that happen. The unexpected results of that act of kindness will take him to a clinic where other things will happen which will start him on a path of recovery that he never anticipated and possibly did not even want, although he desperately needed it.