Who is The Monkey (Mary Jane Reed) from Portnoy's Complaint and what is their importance?

The Monkey is the nickname Alex gives to a woman with whom he has an intense, and at times almost romantic, sexual relationship (for a possible explanation of how she got her nickname see Chapter 4 Part 4 Analysis). She is lower class, crude and uneducated, expresses her feelings with raw passion, and confronts Alex with uncomfortable truths about himself and his behavior—truths which the reader can see as such, but Alex apparently cannot.