Historically, English used "he" as both the male gendered pronoun, and as the "unmarked" or generic pronoun, to be used when the gender was unknown, or for a fictional person such as the plaintiff in a hypothetical lawsuit where no gender had been assigned. This "rule" dates to the mid-18th century and was widely prescribed during the 19th and much of the 20th. Today, many people consider this usage offensive, and it is rapidly declining. I would advise against it.
In response to this, invented pronouns such as "Xi" and "Zie" were created, but none of them ever gained widespread usage, and few people continue to use them.
Many people now use "singular they" this was in frequent use as long ago as the fourteenth century, It was used by Chaucer and Shakespeare among many other well-known writers. Using this is unlikely to offend or confuse, but some still dislike it, and it does make grammatical rules about number agreement hard to follow in some cases, and can in some cases be confusing.
One can often avoid pronouns altogether, using the character's name or description directly. This will often give the writing an odd flavor. This may be desired to highlight the non-gendered nature of the character, or unwanted as drawing undue attention or breaking the flow of the writing.
I have seen works where the author alternates chapters, calling a person of unknown gender "she" in the first, chapter, "he" in the second, "she agaion in the third, and so on. I found this both confusing and affected.
I recall one popular SF series in which female characters referred to people of unknown gender as "she" while male characters used "he". This seemed too subtle to make its point, but might work for fiction set in a society quite different from our current one. So might the use of an invented pronoun.
The choice for a writer depends on the desired effect. If one wants to sound like a traditional writer of say 1750 to 1960, use "he". If one wants to sound like many current writers as of 2021, use "they". If one want to emphasize the difference of a specific agendered character from other characters, use an invented pronoun or avoid pronoun use completely for that character. No one choice is best for every destined effect and audience.