Megan is Rufus Sixsmith's favorite (and maybe only) niece. They see movies and talk about science together. She's like the daughter he never had. She's also one of many deus ex machina plot devices driving Half-Lives: The First Luisa Rey Mystery. Considering that this part of the book is supposed to be a hack spy novel, it's not surprising that she happens to show up at the exact time Luisa is most in need of the Sixsmith report and—gasp—she knows exactly where it is.
Megan's also Hawaiian, which helps ensure that almost every section in Cloud Atlas has some mention of Hawaii in it. No, really: they almost all do. Go find the references—it's like finding Waldo in a grass skirt.