The demands of profiling in Criminal Minds typically keep the BAU team members from being able to sustain happy relationships, but Jennifer “JJ” Jareau was the one exception, together with her husband Will LaMontagne Jr. since season 3. While having a loved one can be a liability - like how Maeve Donovan was used against Spencer Reid - because Will and JJ were both field agents, they found a way to make each other stronger. He was introduced to the characters in Criminal Minds all the way back in season 2, so Will has been a supporting character on the show for almost its whole run.