Monica (surname unknown) is a minor character in the Fault in Our Stars, and at the start of the book is Isaac's girlfriend. She later dumps him upon learning that he is going to become blind.
Before dumping Isaac, when Hazel and Augustus watch them making out in the parking lot of the Literal Heart of Jesus, they repeatedly tell eachother 'Always'. Augustus explains that this is a sort of lover's motto that they share, and text each other constantly.
Later on in the book, Augustus, Hazel and blind Isaac go to Monica's house and egg her car, whereupon her mother exits the house, and Augustus cleverly tells her to go back inside, which she does.
After this, Monica is not mentioned again in the book.