Missed me?
After being trapped in Dipper's mind, Bill realizes that maybe threatening and killing the boy wouldn't be as entertaining as manipulating him. After all, his family had set up the perfect board for him, all he had to do was move a few pieces. At some point that neither Bill nor Dipper could pinpoint, the two become obsessed (in different ways) with each other. Bill loved the dependency of Pinetree on him. And Dipper loved the attention he got from the triangle. In the end, things start to drastically shift in the teen's mind and his new best friend is there to guide him.
Mabel isn't the best sister, but she tries. It's not like Dipper's the best brother either. They both have messed up ways of dealing with their trauma, but she doesn't realize just HOW messed up her brother has become.
{Please note that this book contains graphic depictions of abuse, strangulation, trauma responses, and hospitalization trauma, along with dark humor, non-consensual medical treatment, death etc. Reader discretion advised.}
(This may not be accurate - it is fanfiction. Also, The Book of Bill is not canon in this story. I advise reading the tags. Lastly, the show and characters that are in the show don't belong to me. They belong to Alex Hirsch and the team.)