Thomas "Tommy" Edward Rizzoli is the youngest child in the Rizzoli family. He is often seen as the "screw-up" among the Rizzoli children.
When he was 29, he was arrested for running over a priest at a crosswalk on his third DUI and had to serve three years in prison for it. He then returned to his family and the family fold following his release from prison.
As of He Ain't Heavy, He's My Brother, Tommy is 32 years old.
Tommy was to be released early from prison in "When the Gun Goes Bang, Bang, Bang" for his good behavior. Angela and Frank decided to throw him a party, one that they did not tell Jane and Frankie about. Jane and Frankie still haven't gotten over the fact that their brother is troubled and needs to change, so they tell their parents that they will not be at the party to welcome him back. Tommy later calls his parents, saying that he doesn't feel that his family would welcome him home, so he does not come to meet them, leaving Angela heartbroken.
Apparently, Tommy got into a fight at the prison yard, so he was put back into custody. He has also apparently been exchanging letters with Jane prior to the beginning of Season 2.
Three months later, Tommy is released early again. He finally appears in "Don't Hate the Player" when he arrives in Maura's guest house, where Angela had been staying in since she and Frank lost their house, while Jane was there. Jane, not knowing that her brother has been back and is staying in the guest house with Angela, is surprised, though is somehow happy to see him. Despite this, both Jane and Frankie still think Tommy will screw up again.
Meanwhile, Tommy, noticing that Jo Friday has been licking her paws too much, takes her to the vet, either with worry or in an attempt to show Jane that he has changed for the better. However, he ends up infuriated and immediately calls Jane when the vet charges him $300 after diagnosing that Jo is depressed and needs Prozac, saying that the cops had been saying the same thing about him before.
Later, when Maura's TV set and some of her jewelry goes missing, Jane immediately comes to the conclusion that Tommy had already gone back to his stealing habits. Frankie and Jane find Tommy buying drinks for a girl in the local bar and confront him about it. Tommy then tells them off, saying that they blame him for everything, and he understands that, saying that he feels like the screw-up child of their parents and the reason why their parents have been fighting and are getting a divorce.It turns out, Tommy has been walking dogs to earn money. When they later find out that it was in fact a burglary ring and not Tommy who took Maura's things, Jane is relieved that her brother did not do it and she apologizes to him.
In "Gone Daddy Gone"
In "Dirty Little Secret", when their father Frank returns, they reveal that he and Tommy have been meeting when Frank was in town, and that they did not tell Jane and Frankie because they would judge him. It is also revealed that during one of their meetings, Tommy ends up introduces Frank to his old girlfriend, Lydia, who then ends up as Frank's girlfriend and fiancée. When he later goes to Maura for advice if he should tell his dad that he and Lydia used to go out, he ends up flirting with her since she and Jane, who was the reason Maura wouldn't date him, weren't talking.
After he gets a job as a pick-up guy for Murphy's funeral home, he ends up being hired to pick up a body of a man who was found floating and dead that, coincidentally, Frost was investigating with Tommy's brother, Frankie. He then throws up at the sight of the body, and he and Frost puke together. After that experience, he quits the job.
In "Stiffed" Tommy comes to visit with TJ. He then tells Angela that Lydia has met someone else. He says it's for the best as he feels he is better at being a father than a husband. Angela suggests him to move back and that she would help him with TJ. Over pizza he discusses this with Jane and Frankie and Jane offers to talk to the professor about him using the town house. The professor agrees and Tommy decides to stay in Boston with TJ.
Initially, Jane and Frankie described him as irresponsible and as a troublemaker as he is the only Rizzoli that screwed his life up. As the series progresses, however, Tommy managed to prove that he has a sense of responsibility and has begun acting his age. Furthermore, Tommy is caring and honest, although he often seems to be at the wrong place at the wrong time. He has low self-esteem because of his criminal past and the mistakes he has made throughout his life. After he was released from prison, though, he has been dedicated to getting his life on track. Since the birth of his son Tommy Jr., Tommy has wanted to be the father that his son deserves.
In Season Two he is living with Maura and his mother Angela. He is seen flirting with her in "Gone Daddy Gone" while playing a game of Chess.
Finally in "He Ain't Heavy, He's My Brother" he finally shows her he has feelings for her. She didn't want to be in a relationship with him due to the fact that he is Jane's younger brother.
She's the mother of his child TJ.