>>281567306
3. Reference to gender by characters outside of Haku’s present role. For example, Haku’s flashback in the anime presents Haku’s mother. If the mother were to call Haku a girl, then that would settle the issue, most likely. The same may not be true for the alternative reference of “boy” since it is a flashback being told by one who uses “boy” for self-reference. It would depend on how the flashback is portrayed. If it comes from Haku’s point of view and is told to someone else, then it could be altered in that way to support Haku’s present day role of being a boy. If the flashback were actually the past being told by the series itself or through narrative style (retcon), then any reference would be canon and correct. Also, if Haku were to remember the reference through introspection and not through revealing it to anyone, then any reference would be canon and correct.
Unfortunately, the flashback features the mother calling Haku a “child” and not identifying her as either boy or girl. Future episodes or issues may contain flashbacks such as these.

4. Clear statements by the writer of the series. Obviously he is the one to listen to if you want to know something about a series.

Now, I have found at least two of these that prove Haku was most definitely a girl, but like I said, I will first disprove the popular theory.