I'm not sure specifically as I haven't read the 'left behind the novel'. But typically, in a novel with a title like that, there might be a character who is the one left behind. It could be a child, an adult, or even an animal in some cases.
One way is to simply ask them. Have an open and honest conversation about how they became friends. Most people are happy to share their stories. Another way is to observe their interactions. Notice how they react to each other in different situations, if they have inside jokes or special ways of communicating. You can also look at their shared memories, like old photos or mementos they keep. These can give clues to the story behind their friendship.
It's a story about two people who have some kind of conflict or obstacle preventing them from being friends. Maybe it's a past history, different goals, or opposing personalities.
In the 'left behind film story', there could be a family man who is left behind and has to take care of his family in the new, dangerous world. Another main character could be a journalist who is trying to uncover the truth about what has happened. And there might be a religious figure who helps guide the others through their spiritual journey during this trying time.
It depends on the specific game. In some cases, it could be a secondary character from the main game who becomes the focus in the DLC, like Ellie in 'The Last of Us: Left Behind'.
If it's a story about a sports team, the players are the main characters. They support each other during tough competitions. For instance, if a player gets injured during a game, the others might adjust their strategy to protect the injured one and make sure they all cross the finish line together, metaphorically speaking. This shows that no one on the team is left behind.
Since I haven't read the novel, I can't say for sure. But I assume the girl who was left behind is one of the main characters. There might also be the people who left her, even if they don't appear much in the story, they could still be important as they are the cause of the girl's situation.