Unityでカメラの向き(ベクトル)を参考に移動するソースコード

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class PlayerController : MonoBehaviour
{
    [SerializeField]
    Transform mainCamera;

    float scale = 0.1f;

    // Update is called once per frame
    void Update()
    {
        if (Input.GetKey(KeyCode.W))
        {
            var direction = Quaternion.Euler(mainCamera.eulerAngles) * Vector3.forward;
            transform.position += new Vector3(direction.x, 0, direction.z) * scale;
        }
        if (Input.GetKey(KeyCode.S))
        {
            var direction = Quaternion.Euler(mainCamera.transform.eulerAngles) * Vector3.back;
            transform.localPosition += new Vector3(direction.x, 0, direction.z) * scale;
        }
        if (Input.GetKey(KeyCode.A))
        {
            var direction = Quaternion.Euler(mainCamera.transform.eulerAngles) * Vector3.left;
            transform.localPosition += new Vector3(direction.x, 0, direction.z) * scale;
        }
        if (Input.GetKey(KeyCode.D))
        {
            var direction = Quaternion.Euler(mainCamera.transform.eulerAngles) * Vector3.right;
            transform.localPosition += new Vector3(direction.x, 0, direction.z) * scale;
        }
    }
}