A boggart is a magical creature that takes the form of whatever the viewer fears most. The Sinestro Corps is all about inspiring fear. Thus, it makes sense that a boggart would be recruited to the Corps.
The great thing about the Green Lanterns, Yellow Lanterns and Red Lanterns with the interchangeable heads is that you can switch heads around between the different Corps, which is how we ended up with Medphyll's head on a Sinestro Corps body. He had to color-correct the hands to make them match the rest of his exposed skin, and also turned his face green so he'd look like broccoli instead of a bonsai tree. We named our new character Broccoli Rob.
Harry comes from NECA's "Year Two" set. And yes, he should either be using "Expecto Patronum" or "Riddikulus" to defeat a boggart, but instead he's using that favorite old standby spell, Explodio Face.
The title of this one is a reference to Shakespeare's All's Well That Ends Well, Act IV, Scene III.