Mabel Pines is Bisexual. She's had a crush on Pacifica Northwest since she was 13 but didn't know it at the time. They've stayed in touch ever since Mabel left Gravity Falls.
A/N- I added a picture to describe what Bill was looking at in chapter 2 if you want to look at it. I also added to chapter 2 more to Bill's fantasy.
Also I'm adding a description to the Dipper and Bill chapter for each of them.