Usually referred to as "Junior" or "Uncle Jun" (pronounced “June”), he is the de jure boss of the DiMeo crime family for most of the series.
Dominic Chianese's performance as Junior received critical acclaim, garnering two Emmy nominations for Outstanding Supporting Actor In A Drama Series.
He tells Tony that he admired John F. Kennedy but didn't like his brother Robert because of the latter's involvement with the United States Senate Select Committee on Improper Activities in Labor and Management, as well as the prosecution of Jimmy Hoffa and investigation into the International Brotherhood of Teamsters.
He recounts fondly teaching Tony body surfing on the Jersey Shore as a child, and purchases AJ a $400 surfboard for his birthday.
Junior was in love with Fran and wanted to propose, even buying a ring for her, but couldn't allow a marriage to affect his status in organized crime or summon the courage to ask her.
He fled to Boca Raton to avoid any possible repercussions, but Tony and Soprano soldier "Big Pussy" Bonpensiero (Vincent Pastore) arranged a meeting to solve the problem.
Sometime later, Junior attends the funeral of "Buddha" Bonpensiero, a made man killed by Harold McBrayer's African-American mob in a gangland hit.
In the opening episode of the show, Junior plans to kill "Little Pussy" Malanga at a restaurant owned by Tony's childhood friend, Artie Bucco (John Ventimiglia).
Tony makes many attempts to prevent the murder and eventually resorts to fire bombing the restaurant to force its closure so the hit would happen elsewhere, enraging Junior.
Tensions escalate further when two of Tony's criminal associates Christopher Moltisanti (Michael Imperioli) and Brendan Filone (Anthony DeSando) hijack a truck owned by a Harrison, New Jersey, company that pays protection to Junior.
Upon Jackie Aprile's death, Tony has the support to take over the family but fears Junior will start a war if he is not named boss.
Tony resolves the situation by letting Junior become boss in order to steer law enforcement attention away from the rest of the family.
Tony retaliates by having two of Junior's enforcers, Mikey Palmice (Al Sapienza) and Chucky Signore (Sal Ruffino), murdered.
Along with a bigger share from his old high-end poker game, union rackets, and car theft ring, this enables Junior to live on a subsistence level while also making enough to pay his legal fees.
This leaves Junior the senile Murf Lupo (Val Bisoglio) as capo, Beppy Scerbo (Joe Pucillo), and Bobby Baccalieri as soldiers.
While he is under house arrest, Soprano captain Richie Aprile (David Proval) is released from prison after serving ten years, and actively seeks Junior's friendship.
Junior is conflicted over which side to favor but eventually decides that while Tony can be selfish and impulsive, Richie simply is not respected enough by the rest of the family.
Although Junior has survived cancer and possible prison time, the toll of a series of 'mini-strokes' and the confinement of house arrest has left him confused, depressed, and in failing health.
Tony refuses to put his uncle in a nursing home, feeling obligated to care for Junior himself with the aid of his sisters and Bobby.
Junior is arrested and taken into federal custody over the shooting, but his lawyer secures him a release into a cushy mental institution, claiming he is currently unfit to stand trial.
Within the confines of the mental home, Junior behaves like a typical Mafia chieftain; bribing orderlies, organizing card games, and even physically abusing a rival.
A young patient named Carter Chong looks up to Junior as a mentor and father figure and admires his aggressive, imposing, and rebellious nature.
In the episode "The Blue Comet", Janice (Aida Turturro) approaches Tony and tells him that Junior has run out of money, and will be removed from the Wyckoff therapeutic center.
Tony shows no sympathy for Junior and offers no support, telling Janice that she and Bobby are cut out of his life too, although it is just an empty threat.