He's not officially in the games. There was a short comic that was drawn by the fine artists at Valve. The comic takes place during the end of the first game and before the second game. It shows a man named Doug Rattman who was a scientist at apature labs during the creation of GlaDos, when she went mad and tried to kill her creators. He has forever been hiding in the walls of the labs and is the one who leaves all of the paintings. Look up "Lab Rat" online and read the comic. It gives you much more detail about the character. You can also hear his voice rambling in the walls in Portal 2.