@GoldenXan Well there is that flashback scene when you're in Prodigal's mind. When you find yourself gripping something and choose yourself, you find Prodigal admirig a hologram of you and well Ill just pull the sequence from in game:
You find yourself staring face-to-face with a life size holo-projection of yourself. Beside it, a teenage Prodigal sits studying the image.
"Someday, you'll know how meant to be, how connected, how intertwined we are," Prodigal sighs. "And you'll pay for what I've lost with all you've gained."
You feel a profound sense of longing, staring at yourself. One you're pretty sure doesn't actually belong to you….
Now looking at this for context clues, it has to be before the Hero Project because she wasn't out to take you down then. By that point she'd figured out Miss Artillery was alive, at at least so far as I can tell as she was monitoring the conspiracy and tried to warn you. So the absolute earliest this memory could take place is before the Death Wave.
However, unless I'm totally off base here, it quite possibly takes place even earlier than that. The gap between the first and second games is seven months. With the gap between the Hero Project finale and Herofall, at best these three games' events take lace over the span of a year. Now in this memory Prodigal is referred to as "teenage", implying that she's not a teenager in the present, otherwise this would not be notable. Again these games takes place over a year, likely less given the possible ending in Herofall where you are still the reigning Hero of Tomorrow, an award that is given out annually.
Now the important part is that sense of longing. Longing isn't about vengence, in fact its quite the opposite. It's desire by definition. Not to mention she sighs, a clearly romantic action.
So all in all this memory has to take place before the first game. Now before playing Herofall the thought that Prodigal was in love with the MC never crossed my mind, but Herofall retconned this into her character, and as such it colors those interactions with her, both as Prodigal and Sparrow looking back. Does that make sense?