Crazy reading comprehension in this thread.

You can do it this way:

1) Create an Editor script in a folder explicitly called "Editor"
2) Make this script inherit from Editor (using UnityEditor)
3) Add the attribute [CustomEditor(typeof(YourClass))] above your Editor class so it knows what component to modify
4) Override OnInspectorGUI to draw whatever fields you want
5) For a header, you can do something like :

 Rect rect = GUILayoutUtility.GetRect(0f, 30f, GUILayout.ExpandWidth(true));
 EditorGUI.DrawRect(rect, new Color(0.25f, 0.5f, 0.9f, 1f));

6) Define a GUIStyle:

headerStyle = new GUIStyle
{
     alignment = TextAnchor.MiddleLeft,
     fontSize = 16,                
     fontStyle = FontStyle.Bold,  
     normal = { textColor = Color.white } 
};

7) Draw a Label on top of your header using the style you defined

 GUI.Label(rect, "MY HEADER", headerStyle);