There's no explanation behind how you arrived in the Harry Potter world and took over the body of the main protagonist. What you did know, your foreknowledge was enough to save Hermione from the troll and be her best friend. Now, the fate of the Wizarding World may rely on the two of you.

https://gildedshivers.tumblr.com/post/624551116318834689/hermione-granger-portrait