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(Maria's POV)
At the end, I didn't have to intervene, and Sofia was surprisingly able to handle Martina all on her own and win against her, even when the girl was growing more corrupted by the Nightmare Powers she was beginning to develop.
Sofia's true power, I can tell what it is now, as someone that survived becoming a Nightmare and emerged from it with something new, the Mechanical Heart…
She's someone destined to clean people's hearts that had "darkened", the light that her Mechanical Heart can produce is capable of extinguishing this darkness, these nightmares, and clean a person's mind.
I don't really know the complete reasons behind why Martina hated Sofia so much though, but from what I heard as they battled, when Sofia was much colder, she had done something to Martina that led to her being bullied constantly for a long period of time.