Events of American Dreams
After Marlene, Riley and the surviving Fireflies kill the Bandits that ambushed them in their hideout, Trevor made himself responsible for holding Ellie back when Marlene threatened Riley with a gun when she insisted to join their cause. When it appeared that Riley was danger, Ellie bites Trevor's hand and steals his gun.
Events of Left Behind
Sometime after their encounter, Riley came across Trevor in the quarantine zone. Aware that he was being followed, he lead Riley into an alley where he and several other Fireflies ambushed her. They took her to their hideout to meet Marlene, who orchestrated Riley's encounter with Trevor.