Referenceと同じ回転量を適用

CopyRotation.cs

using UnityEngine;

public class CopyRotation : MonoBehaviour
{
    public Transform Reference;
    Quaternion rotator;

    void Start()
    {
        rotator = Quaternion.LookRotation(transform.forward, transform.up);
    }

    void Update()
    {
        transform.rotation = rotator * Reference.rotation;
    }
}