St. Jimmy is the alter ego of Jesus of Suburbia, a character introduced earlier in the album. It's debated whether St. Jimmy is an actual creation of his or a split personality. He seems to be much tougher and less idealistic than Jesus. The album lyrics are written in diary form. The song deals with the initial development of this new persona, beginning with his arrival in the city, as described in the eponymous 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 involvement in crime: a mixture of drug use, vandalism, petty theft and all-around violence. Jimmy is featured prominently in the remainder of the album. He meets Whatsername, who he gets in a relationship with, as told in "She's a Rebel" and "Extraordinary Girl". His strong feelings for Whatsername create an inner conflict between his new lifestyle and his responsibilities. In "Letterbomb", he is reminded by Whatsername of how he used to fight for the city, and that it's 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 moment when Jesus of Suburbia became Saint Jimmy
Jimmy also appears in the official music video of "Jesus of Suburbia", portrayed by Lou Taylor Pucci. The main character, possibly representing Jesus of Suburbia in the video, writes Saint Jimmy in blood on the wall of a bathroom. The character is evidently a delinquent, and leaves town after his misadventures come to a halt. 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.
Also in the Jesus of Suburbia music video, it is shown that he may have issues with his mother, since he ran away from her, this could also be the fact that he has depression, since Whatsername left him, he went out with multiple different girls and cuts himself. So he could very well be running away from his problems.
In brief, Saint Jimmy is one of the main characters of American Idiot, along with Jesus of Suburbia and Whatsername. Jimmy's true nature is open to the interpretation of the listener; some consider him a discrete, separate entity created by Jesus of Suburbia, while others think he is simply a result of SPD.
In the music video for "Wake Me Up When September Ends", the relationship between the two characters may also point to the relationship between St. Jimmy and Whatshername. even though they have different actors its not entirely improbable considering the nature of what he does as a job may also point to a revival in St. Jimmy.