This is caused by faces inside your mesh. The main sign of this is that the crease only appears when subsurf is turned up; the inside faces add weight that pulls the verticies inwards (according to the subsurf algorithm).
Tab into edit mode and uncheck the X box in the Mirror modifier. Your properties panel should look like this:
This will leave you with half a human. Change into face select mode: and select these faces:
Key X and select Faces. Tab out of edit mode and recheck X in the Mirror modifier. This is the repaired mesh:
I had a similar problem after applying the mirror modifier (no crease but a sharp line between mirrors). What worked for me was to select all and use Mesh - Normals - Set from faces in edit mode after the modifier was applied.