Who is the Ratman?

There are secret rooms in some chambers that have graffiti in them. In a comic released by Valve, there was a man named Rattaman, whom saved Chell between the first and second game. He taught Chell and painted all the graffiti. Those rooms with the graffiti are known as the Ratman rooms. Here is the link to the comics:
www.thinkwithportals.com/comic/

He's some schizophrenic guy that used to work for aperture and he helps chell escape