CODE EXAMPLE FOR CSHARP

unity customize hierarchy window

using UnityEditor;
using UnityEngine;
[InitializeOnLoad]
public class CustomHierarchy : MonoBehaviour
    static CustomHierarchy()
    {
        EditorApplication.hierarchyWindowItemOnGUI += HandleHierarchyWindowItemOnGUI;
    }
    private static void HandleHierarchyWindowItemOnGUI(int instanceID, Rect selectionRect)
    {
		//code to edit the hierarchy here        
    }
}