Hmm... Lol! That's a hard question to answer. I debated between three characters. I eliminated one because it would give spoilers and tossed a coin in my mind for the other two.
I'll go with Em.
Em has been sucked into a totally new world... After steadily get sicker and sicker for three years, expecting to die any day but stubbornly holding onto life, she thinks her sick brain is playing tricks on her and she's in an extended coma. At first, all she wants is to wake up and say goodbye to an older sister who'd taken care of her for years.
But she's too full of life to just sit around!
She begins to enjoy every minute in her well, slightly younger body and starts to forget the other body... Except the rare occasions she's sucked back.
Because she thinks she's in a dream, she does things that she normally wouldn't do. Like being straightforwardly honest more often (not 100% just more often. She knows when it'll be dumb for her dream happiness). Just to see what would happen.
She's not an over the top character, but she has meltdowns from the overwhelm of events, joyful moments when things are just beautiful, screeching anger and frustration when she can't help the ML more, tantrums she'd never allowed herself before...
She slowly regresses into the child she wasn't allowed to be as Em and grows up happier.