Rusty was finally free from the Chaos Council, and finally free to do as she wants. And with her new found sisters beside her, there's nothing she couldn't do.

As everyone was fighting the grim robots, Rusty noticed you were being ganged up by a few of the grim Amy's. So she came to help you, after all she owed you for helping her realize she's not just a robot. And that she deserved happiness and freedom.