which software you are using for this character model ?
anyway in Maya -
This types of problems happen due to double edge at hard face ( Extrude Subdivision is more than 1)
For this you can follow this step-
1.select all the vertices and < merge vertices
3.Press 3 on keyboard to check smooth toggle on and check your Hard line is looks fine .
4.Go to Normal>unlock Normal>Soften Edge
4.Export it to .Fbx