Jimmy usually hangs out at his clubhouse. He likes to smoke cigarettes and is sometimes accompanied by his two cronies. Jimmy likes to bully Bill because he is known to be a pussy. He got in a fight when Bill punched him, causing his wood shops project to break. Jimmy's father works at the school as a janitor.
In "O Holy Moly Night", Jimmy explains that one time, the milkman refused to give him some heavy cream. As a result, he stuck a pencil in his leg, which resulted in him becoming an altar boy at the church, even being promoted to head altar boy throughout that time. When Bill first became an altar boy himself, Jimmy immediately trained him in the ways, and also revealed that he was respecting Bill's recent display of courage at standing up to him. Though, he inadvertently goes back to being Bill's enemy when he refused to help steal the golden chalices from the church, and after Bill accidentally spilled wine on his surplice. Later, he and his father confront the Murphy's because of that indiscretion, which ended after Sue hit Jimmy's father with a baseball bat.
In Season 2, Jimmy is still shown to be overly harsh and bullying towards younger kids. Halfway through the season, Bill steals a hockey stick and blamed it on Jimmy (since he's the one who took a fraction of the money before Bill could purchase the stick). As a result, Jimmy's father sends him to a Catholic reform school to straighten him out.
Although his father's check bounced after a few weeks, Jimmy undergoes a major personality transition and sets off to make amends for those he hurt, including Bill and Phillip. Unfortunately, when Jimmy was apologizing to Phillip, he unintentionally tells him that Bill's the one who sent him away in the first place, causing Jimmy to change back into his old self. This in turn helped Bill and Phillip mend their friendship after they had a falling out.
The two of them along with Ben and Kenny then decide it's time they gave Jimmy something he deserves and scheme up a major plan to ruin him. However, the plan goes south when Jimmy wises up on the four, but they end up getting satisfying results anyway when Randy hits Jimmy with his van. As Ben, Kenny, and Phillip run off, Bill single-handedly takes Jimmy to the hospital to get his injuries treated.
After this, Jimmy thanks Bill for the deed, but he still admits that he'd do something much worse if it was the other way around and will no doubt give Bill a major beating once he recovers. During this time, Bill meets Jimmy's sister and instantly becomes smitten with her.
Jimmy is like any other bully, constantly picking on weaker kids and treating them like crap. He is also known for being a liar, when he told his dad that Bill was the one who planned to rob the church, even though it was really him.
Despite his tough guy attitude, he seems to have a fondness for anyone who "has balls", which was why he didn't do anything bad to Bill for assaulting him and for burning down the city's forest.
Jimmy is also a skilled craftsman, as evidenced in "Bill Murphy's Day Off" when he easily built a birdhouse that got him an A in wood shop class, and later on when he built a wooden boy he named "Peppino". This reveals a secret softer side of Jimmy, as also evidenced by the various stuffed animals in his bedroom.
In Season 2, Jimmy is still portrayed as aggressive and mean. However, when he's blamed for stealing, his father takes him to a Catholic Military School, where despite only being there for 2 months (he's released after his father's cheque bounced), he undergoes a major personality transition, becoming a much nicer and more caring person. This doesn't last very long when Phillip tells Jimmy that Bill's the one who framed him for stealing, prompting him to go back to his bullying ways.