How do I make a character walk backwards, rather than spinning to face the camera?

This is my first project in Unity. I am using the script as below (section of), with a simple Mecanim animator controller. It works fine - but I want to modify the character behaviour. As it is, when I press the up arrow, the character walks forward along the Z-axis away from the screen, and when I press the down arrow the character flicks around (no smooth movement) and walks toward the screen.

I'd like to ask the following questions:

--- Why does the character flip around? How is the target direction calculated?

--- I have a good backward walk animation. How would I modify the script so that on pressing the down arrow the character remains facing forward but executes the walk back animation?

Any help much appreciated.

------------ script fragment----------------

 void Update () {
     // Getting Input
     _hDirection = Input.GetAxis("Horizontal");
     _vDirection = Input.GetAxis("Vertical");
     
     #region Camera Transform
     //Get Main Camera Transfrom
     Transform cameraTransform = Camera.main.transform;
     
     //Get forward direction of the character
     Vector3 _forward = cameraTransform.TransformDirection(Vector3.forward);
     _forward.y = 0f; //Make sure that vertical direction equal zero
     // Right vector relative to the character
     // Always orthogonal to the forward direction vector
     Vector3 _right = new Vector3(_forward.z, 0f, -_forward.x); // -90 degree to the left from 
             the forward direction
     //If we are moving backward
     if (_vDirection < 0) { _isMoveBack = true; }
     else { _isMoveBack = false; }
     #endregion
     
     #region Calculating target movement
     //Get target direction
     Vector3 _targetDirection = (_hDirection * _right) + (_vDirection * _forward);
     
     //If the target direction is not zero - mean there is no button pressing
     if (_targetDirection != Vector3.zero) {
         //Rotate toward the target direction
         _moveDirection = Vector3.Slerp(_moveDirection, _targetDirection, rotationSpeed * 
                      Time.deltaTime);
         _moveDirection = _moveDirection.normalized; //Get only direction by normalize our 
                                                                    target vector
     } else {
         _moveDirection = Vector3.zero;
     }

----------------------------End of fragment-----------------------

Thank you.