was following a tutorial where I stumbled into this weird line in my mirrored mesh (sorry for the feet lol)
You can also download the blender file
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Problem
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).
Solution
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:
