Concept art sketch
Concept art sketch. I tried to integrate the elements you mentioned by emphasizing the main character and the girl who can be seen through the glass of the car. for Lizzie-Lou, I left the outer shape of the sticker in the shape of the two pigtails to reference what she was before. I used purple as part of the background that mixes with blue in such a way that a transition is generated between the two colors that is perceived as the reflection of the sky in the window, I like it that way since blue psychologically brings that charge of calm and confidence while purple carries a spiritual burden that falls not only on Lou but also on Elizabeth. Also, in the upper left corner you can see the mountains in the distance. I hope you like it and any details you want to integrate or remove, let me know :)
