How do you get the half demon half angel Lillie's gc scene

Maybe you haven't unlocked the strategy battle system yet? Return to the old barracks where you find Mira, after repairing them via the overview map. There you'll find Jessy and she'll give you the quest to unlock the system.

I did but after I unlocked it all she did was give the dialog (I look forward to going into battle with my own squad)