Hear me out, and get ready because this'll be a doozy.
Marcy is the one responsible for saving everyone, all thanks to a long term domino effect that started here.
Ok, ok, listen listen.
When Marcy met Andrias (as shown above) her cheerful personality endeared her to him, and because of this Andrias doesn't kill her right then and there like The Core wanted instead suggesting the Three Temples scheme.
A lot of time later, Marcy reunites with Anne, and Andrias starts the temples plan, and to go to the temples Marcy learns the ancient amphibian language.
Then she and Anne do the Temples and learn the lessons, and during the second temple Marcy unintentionally keeps Anne from fully loading the gem.
They meet back up with Sasha in the Third Temple and later The True Colors incident happens, Anne discovers her powers and Marcy activates the Box allowing Anne to escape to Earth, which of course leads to her getting impaled.
BUT! Here's the thing:
This gets Andrias distracted allowing Sasha and Grime to escape and form the rebellion, and more importantly Andrias actually regrets doing it.
On Earth Anne can have time to decompress and reflect back on everything, learn to use her powers at least a little, and discover Leif's last clue, and thanks to Marcy's notes on the Amphibian runes, which led her to The Mother of Olms.
Back with Marcy, the regret of hurting her starts to affect Andrias and he starts second guessing himself, when The Core posseses Marcy the regret grows stronger.
Fast foward to All In and Leifs letter makes it back to him, and when the letter talks about Andrias learning to love again it is Marcy who comes to mind.
Leading to Andrias losing the will to fight and finally let himself be defeated.
(Wow, actually looking at a still of it, that really was a gruesome blow)
AND, because he refuses to fight Darcy gets distracted from their fight with Sasha, giving her the opening to cut the wire connecting the Core to Marcy, freeing her.
Heck, once Andrias finally realized the error of his ways and decided to stand againt his father and the Core, that could also be tied to Marcy's influence.
Look back to all of it.
Every key moment where things could have been over for the stars it was Marcy who unintentionally kept things how they should.
Sometimes it felt like fate conspired againts Marcy, but at the same time it looks like Marcy herself is an agent of fate that allowed for most of the major developments that lead to the Prophecy being fulfilled.
Everything would have gone to hell in a hand basket if not for Marcy.