Kerry Cerberus is imprisoned in Jerry's mind as an upper working class girl with aspirations to enter into middle class society through her education. He is a projection of what she will become, and in him she recognises her own faults.
She walks up the stairs to his room, hypnotically drawn to her quest to interrogate him. It represents her crossing the line from being an initial ally to evolving into an enemy. She invades his room, sussing out an emotional weak spot in his character and starts to needle him. She continues to belittle him, judging him by the standards of the culture she wants to enter into. But in passing judgement about him being unemployed she has condemned herself.