After I saw this, my mind went on a slow drive. This does sound like the plot for an anime: a geek befriends a new girl they do something and her secret gets exposed.
The next tangent sent me on who this would be a good plot for in current fanfic. The guy in the picture--not the one in the shadow at the end of the tunnel below her tail--reminded me of two people: Kensuke Aida and Harry Potter.
So the idea? Harry has a friend he is close to, who doesn't run from Dudley.
Honestly, if she was there to hide, then something far worse than an overweight bully would be her fear.
At some point in time, they get closer, but someone attacks them--could be for Harry, her, or just the random factor of them being easy targets--causing her to reveal her true self.