Finally finished my XCOM 2 campaign which had the Mane Six (plus a few other ponies) in my roster of troops. By the end of the campaign, Rarity has earned the distinction of being the sole survivor of the Mane Six. Here she is overwhelmed with mixed feelings as she is overjoyed to reunite with her sister but at the same time is left mourning the death of her friends.
 
A rough bio I had given her:  
On the surface, Rarity was a fashionista who served the influential elite of ADVENT society. In reality, however, she was a mole who leaked precious intel to local resistance networks. Patrol movements, the schedules for prominent political and media figures, locations of POWs, whatever helped make the resistance life easier. Her luck eventually ran out as ADVENT became suspicious of her activities and she became target. The resistance helped to smuggle Rarity and her younger sister Sweetie Belle out of the cities. They offered them sanctuary at an off-grid refugee camp, promising to keep them safe from ADVENT retaliation. However, Rarity decided that the camp should only take Sweetie Belle while she would join the newly revived XCOM to take fight to ADVENT. Before she left, Rarity had promised Sweetie Belle they would reunite after the fighting was over.