William LaMontagne Jr.

Played by Josh Stewart, Detective LaMontagne was first seen in 2x18 ("Jones") as a New Orleans detective investigating a serial killer case which had initially belonged to his father. Detective William LaMontange Sr. was a detective and had made a breakthrough in the case right before being killed in Hurricane Katrina. The unsub was believed to have suffered the same fate, but when evidence to the contrary arose, Detective LaMontagne took over the case using the work his father had done and a single clue carved on the wall by his father, just before he had died. Detective LaMontagne Jr. called in the BAU to assist him, and they were ultimately successful in capturing the unsub. LaMontagne Jr. spent much of his time on the case choosing to work closely with Agent Jareau.

In 3x17 ("In Heat"), it was revealed that they had been involved for over a year, and in 3x18 ("The Crossing"), JJ called Will to tell him she was pregnant with his child. In the season three finale ("Lo-Fi") Will showed up in New York City while the BAU was on a case, reiterating an off-screen proposal of marriage, and further telling JJ that he would be willing to give up his shield, move to Virginia, and raise their baby. As of early season 4, Will had moved to Virginia, and is apparently now serving as stay-at-home dad to their son, Henry. JJ has not, as yet, canonically accepted the marriage proposal, but they did exchange rings with insets of Henry's birthstone, citrine. He is seen briefly in the 100^th episode getting medicine for Henry with JJ.