As a fellow published author my best advice is to treat them as you would any other character. The backstory isn’t usually the way we introduce our character, it comes later. As others have said, it will depend on their role and importance, as much as their point of transition. Is it important that they be described as such at the beginning or can it be layered within the character arc?
I would suggest spending some time reading the articles on this site and on CDH from the members as they describe their own personal journeys. It will give you a feel for how they think at different points in their transition. Feel free to PM me if you have any other questions.
Managing Editor TGH/CDH