Prevent double serialization of a custom class

Please explain to me how this should be done so that multiple instances of a custom class can be used with in a custom editor as data holders? I.e. I'd like with a PropetyField method have a foldout with a custom class's fields when expanded.

Far as I understand custom class should be parented to ScriptableObject, created with CreateInstance and use HideFlags.HideAndDontSave but then i've got an object picker instead of foldout.

Thanks!