Don’t have to relate to your MC and Non!MC being twins series, but it think it would be so fun to see a parallel between them in power. Now, I’m not too knowledgeable about MC’s past, but from what I can remember, when she was at EVER and being experimented on, every time she died she just came back to life without her memories. Now, imagine reader as a similar power, however, every time she dies, her soul hops into another body, killing them and as reader continues living in that new body, that body slowly begins to mold herself to her appearance and genetics. It’s almost like she never died!
Oooh, this is perfectly angsty idea!
I can see it now: the family of the person you've become noticing things—how “your” tastebuds have changed, how your hair looks just a bit light/darker when the sun hits it juuust right, how you seem lost and confused when someone references an old memory...
They think it’s because “you” had a near-death expereince; that tends to change people. But then, you change too much. And you run away, unable to face the fact you’ve created another ghost alongside making these people mourn their loved one all over again.
Maybe that’s why you become attached to MC? Maybe that’s why you worm your way into the hearts of all those she loved before? Because she cursed a dragon with immortality once... maybe if you break your sister’s heart, she’ll curse you with a permanent death?