Then I made a simple rig for it, and I parented the mesh to the rig. When it came to weight painting, this is how I painted the vertex group corresponding to the neck.
Now I want the head to be able to rotate around the X (red) axis. The issue is, the hair and glasses seem to rotate at a different ratio (faster than the head), angle-wise.
I have no idea why this happens, since the glasses as well as the hair are joined into the character's mesh. I couldn't go with the automatically generated weights because they were messed up.
I'm a newbie to Blender so I expect I do something wrong. I'd highly appreciate any tip regarding this issue.