He is a Supervisory Special Agent in the FBI's Behavioral Analysis Unit, and has appeared from the episode "About Face", which was originally broadcast on October 31, 2007, during the show's third season.

He is shown to be a close friend and colleague of Aaron Hotchner (Thomas Gibson), as they worked together during the early days of the Behavioral Analysis Unit.

He served in the Vietnam War and rose to the rank of sergeant major before retiring from the Marine Corps; based on the backstory timelines, specifically the time in service required to rise to the rank of sergeant major and retire, he presumably served in the Marine Corps Reserve after joining the FBI.E703 In the Season 8 episode "The Fallen", Rossi encounters his old unit commander, Sgt.

He claims to have "written the book" on hostage negotiation, and in one episode steps in as a negotiator when his fellow agents Spencer Reid (Matthew Gray Gubler) and Emily Prentiss (Paget Brewster) are being held hostage.E403 He takes an annual leave to do cross-country lecture and book-signing tours, which apparently attract a lot of female fans, "if Barry Manilow isn't in town.

To date Rossi has been married three times, but he has said the only people he knows how to make happy are "divorce lawyers".E318 In season seven, he reconnects with his first wife, Carolyn (Isabella Hofmann), who tells him she is dying of amyotrophic lateral sclerosis and asks him to help her end her suffering.

Rossi has a romantic relationship with BAU Bureau Chief Erin Strauss (Jayne Atkinson), hinted at throughout the series starting late in season 7.

The full details of the affair are never disclosed, but he reveals his feelings for her when she is murdered by serial killer John Curtis (Mark Hamill), aka "The Replicator"; upon learning of her death, the usually unflappable Rossi becomes deeply emotional.

Despite growing up on Long Island, Rossi is apparently a Chicago Cubs fan, judging by the Cubs gear in his Quantico office.E319 It is hinted that he plays video games, as BAU Technical Analyst Penelope Garcia (Kirsten Vangsness) mentions a report of an abducted child named "Niko Bellic", who Rossi points out to be a character from Grand Theft Auto IV.

In later episodes, it is implied that he is more technically savvy than he was when first seen - in "Epilogue", he attributes his tiredness to a long session of Rock Band and not enough coffee the night before, but he also claims to have "wiped the floor" with Ringo Starr in the process, apparently on a high difficulty level.

On one occasion he has come into a team meeting late and with an undone tie, leading colleague Derek Morgan (Shemar Moore) to ask if he is "working on wife number four".

At times, he has deliberately flaunted his financial security to his superiors - for example, mentioning to Strauss during an interrogation session his grievances about how the price of gold is going up, and having to figure out when to sell.

Also in season seven, Rossi makes his wealth apparent when the team coerces him to bankroll a sting at a poker tournament rather than going through FBI channels for funding.

After going unsolved for 20 years, the case found that a mentally disabled carny clown had committed the murders accidentally when he broke into the house to play with the oldest daughter.

In the season six finale, he is almost shot in the face by a suspect pretending to be a victim of human trafficking; however, Morgan's quick reaction saves his life.