St. Jimmy is the alter-ego of the Jesus of Suburbia - a character introduced earlier in the album. It is debated whether St. Jimmy is a creation of Jesus, or an aspect of a split personality disorder. He seems to be a much tougher personality than the Jesus of Suburbia, and less idealistic. The album lyrics are written in diary form. The song deals with the initial development of Jesus's new persona, beginning with his arrival in the city, as described in the song's lyrics:
"St. Jimmy's coming down across the alleyway/ Upon the boulevard like a zip gun on parade"
The song continues to describe various aspects of St. Jimmy's new "life of crime" - a mixture of drug use, violence and a very rebellious attitude.
The character of St. Jimmy features prominently in the remainder of the album. His story continues as he meets and begins a relationship with "Whatsername", in the songs "She's a Rebel" and "Extraordinary Girl". His strong feelings for Whatsername create a conflict within himself, between his new lifestyle and his responsibilities. In "Letterbomb" he is reminded by Whatsername of how he used to fight for the city, and how it is his duty to continue doing so.
"Where have all the riots gone/as the city's motto gets pulverised?"
He then recounts his deeds and eventually disposes of the St. Jimmy persona in "Homecoming":"Jimmy died today/ He blew his brains out into the bay"
The character of St Jimmy also appears in the Green Day Video for "Jesus of Suburbia" The main character, possibly representing Jesus of Suburbia in the video is called Jimmy and writes his name (Saint Jimmy) On the wall of a bathroom. The character is heavily involved in Sex, Drugs and Crime, and, at the end of the video, leaves town. Also in a conversation that takes place in the video, his mother makes the statement:"What are you? Jesus nailed to that couch, suffering for my sins? And I'm the loser" This is another reference to the character of Jesus of Suburbia."
Basically, Saint Jimmy is one of the main characters, along with Jesus of Suburbia and Whatshername, of Green Day's American Idiot album, whose story is told through the music. Saint Jimmy's character is often left open to interpretation by the listener, as some people think that he is a character created by Jesus of Suburbia, while others think he is a schizophrenic personality.