Our therapist knows, and she seems to think it's a form of "disorganized thinking," which seem to be her nice way of saying "delusion." She probably just didn't want to upset me by being blunt like that, because therapists have to try to be nice about things. Psychiatrists have literally straight up told me they think it's delusional, but they were also close-minded about homosexuality, so keep in mind we're talking about not the most open-minded of doctors. However, I think it says enough about mainstream psychology that any multiplicity, whether natural or intentional, disordered or not, is seen as a negative thing because it's abnormal, and it's sad to see how many people conflate "abnormal" with "disordered."
I'm a psych student, and I don't think it's a disorder given the definition. (Let me get out my notes from my Abnormal Psychology class. Here we go.) There are three criteria for something to be considered a disorder:
Psychological Dysfunction Personal Distress or Impairment Response is atypical or culturally unexpected
Keep in mind that all of these criteria must be met in order to classify something as a disorder.
Clearly, it's culturally unexpected or atypical if you come from a Western country. Tulpas might be considered a form of multiplicity, and I don't see that being accepted here in a long time. Although I do know that tulpas are, like, a Buddhist thing, so maybe it's okay in places where Buddhism is a big thing. I don't know.
There are different components to psychological dysfunction: cognitive, behavioral, and emotional. I could imagine how one could see a tulpa as a sign of cognitive dysfunction, although I don't really agree that it's dysfunctional because if anything we function better than any one of us would on our own (in my system's case).
However, I think the main reason I consider it not a mental disorder is that it does not cause distress or impairment. In fact, I would say that it may, if anything, increase one's functioning as it can help one cope with emotional issues. Niteo even helps me with school. I am better off with him, and I don't know of many cases in which that wouldn't be so (although I would say it's a case by case basis, some people apparently can have issues). Overall, people do seem happier with a tulpa and it may even help them function better as a person.
If anyone disagrees, let me know, but I think I've reached the right conclusion given what I know. It's not a disorder. However, don't expect people outside of this community to understand that. People are silly, even psychologists and psychiatrists.
She can't tell your family anything unless you've signed a release of information to them, so you're in the clear there (unless you have signed one, in which case you can retract it or not tell her.)
EDIT: You'd have to make sure that you actually aren't using it as a replacement for human interaction, otherwise it would be an impairment, and probably unhealthy. Just saying.
EDIT 2: If you're a minor, I'm not sure what the laws/rules are on that. Sorry.