Since we don't know much about his past, I'm writing this as I imagine it. Jay has moments with Mouse, shares things with Erin he hasn't before, and he has time with Will sharing memories and such.
All those years ago, those dreadful long years, during a certain detectives time in the rangers, he was accompanied by mouse and another teammate. The trio were inseparable until the time come when they needed to part their own ways, what happened when they eventually reunited...?
-Chicago PD-