After growing tired of watching humans and robots take her tests, GLaDOS decided to try them herself. She constructed a mobile body for herself and uploaded her consciousness into it. Then she designed another AI to create tests for her, with almost complete access to the Aperture facility. (She also designed a fail-safe in case the new AI tried to take over. She was still paranoid over the Wheatley incident). I know I'm not the first person to make a humanoid form for GLaDOS, but I thought it was a really cool idea.