Cassidy, in the 1930s and 40s, was the life of the party. He was always there with a slap on the back and a "How're yez!" He had a knack for making people love him, and a reputation for using people, especially women. He would start with small favors, little things that were hard to deny him. Slowly he would draw people closer into his web of hedonism and addiction. Eventually the people he loved would be destroyed, but he would go on.
Joanie was one of those he nearly destroyed. Despite what happened with her friend Gilly, she let herself be seduced by Cassidy. By the end of 1945, they were engaged. They were also raging heroin addicts.
In the throes of withdrawal, having done everything he could to try and get his fix, Cassidy tried to fortify himself by drinking blood from Joanie. She escaped from him before he could get his teeth into her, though, and ran to her friend Sally.
In Joanie's absence, Cassidy turned to drinking blood from rats, and was reduced to a corpselike, putrid state.
Sally put her on a bus, and that was the end of her days in New York City.