Hi SinisterF. Since i promised, here.

In this universe, Sunday never learned the meaning of marvelous from her grandma. So basically, she still acts just like her from her childhood.

Going way back in her childhood, Sunday was basically a completely different person. It's not exactly fully known, obviously. But before she found out the meaning of that sweet sweet "marvelous", i doubt that she was just as happy. As a child, Sunday was getting quite a lot injuries and diseases, so she spent a lot time in the infirmary. (Mostly she broke her leg, based from the irl horse)

More to it, she gets her childhood as a nightmare up to this day. (I'll search for the pic later)

So in the end: As a child Sunday was probably happy, time to time sad and tired, spent a lot time in the infirmary, and obviously, DOES NOT KNOW THE MEANING OF MARVELOUS. She doesn't make other people happy, she searches for that happiness herself.

In the original story, Sunday went out of the infirmary being sick and cold, climbed out of the window probably. She almost died (actually she would die if she wouldn't be found), and when she wakes up in the infirmary once more with her parents crying, that's when grandma tells her the meaning of marvelous, that's where she became truly happy. I'll actually drop a guide on what you have to say to turn her into normal self. She won't exactly be the same, but she'll start mentioning marvelous and be happy.

How to teach her the meaning of marvelous and make her happy: "Sunday. Marvelous, is something truly beautiful, a surprise, even if something bad happens, after that something good will happen. Aren't I'm right?"

You won't do bad stuff to her, you won't dare. I'm always watching.