Katniss may be the mockingly, but Rue was their martyr. She started the rebellion.
One death, one moment, one act of defiance, and all of a sudden, she was the reason that the war started.
Her district rioted after her death, not because of how young she was, they had seen children die before, after all- but because of how kindly she was treated afterward.
Katniss showed her true humanity in death, giving her a burial place, making it beautiful, even in the moment as she was dying, she sat there with her and stroked her hair while singing her a lullaby.
Katniss and Rue are intertwined in the rebellion because it could not have happened without EITHER of them. Katniss's actions may have been what fueled the fire, but Rue's death was the spark that lit it.
And it all comes back to Prim.
Katniss was only a part of the games because she couldn't bear to let her sister die, so she volunteered in her place. She took a liking to Rue and treated her with such kindness because she reminded her so wholly of Prim. She killed President Coin, not only because of her idea to begin the games again, but because she knew Coin was responsible for the bombs that killed her sister. She did everything...for Prim.
She stayed alive for Prim. She fought for Prim, and eventually- she mourned for Prim. It made her determined. It made her defiant. Sometimes, it made her complacent, but in the end, it just eventually made her angry. She went back home, having won the fight that she had been trying to win ever since she first got sent to the games, and all she could see was reminders of the one thing that she couldn't get back.
Katniss was their mockingjay.
Rue was their martyr.
But Prim was the reason it all began.