Je dirais on tout simplement, c’est apersonnel et ni masculin ni féminin, comme le they anglais.

À ma connaissance, il n’y a pas de correspondance, de terme officiel en français, mais plusieurs associations, organismes et militant-e-s trans* penchent vers le terme ille (contraction du il et du elle).

My normally trusty source of French stands by her claim that gender neutrality already exists in French and that using the non-feminine forms of pronouns and adjectives is actually gender neutral, so I’m on my own with the following observations (and translation near the end):

I’ve also noticed/used the singular “they” that you mention, but in my experience it generally is introduced/prefaced at least once by using the gender neutral “the/that person” or “the/that individual” before switching to “they/their” (just as you’ve done in the first paragraph of your question: « the person left, but I think they forgot their books »).

The passage about Bry, however, jumps right to “they” without that clear “person”/”individual” warning that generally helps my ears and brain prepare for the singular “they,” which can only mean that my experience is obviously as old-school as my ears and brain. I still feel, however, that setting the stage for singular “theys” by first mentioning the word “person” or “individual” is a defensible middle-ground in English.

French, of course, has its own gender (sexual)-neutral versions of both “person” and “individual” and I think that using a balanced, alternating combination of these two words to replace gender specific pronouns might be a suitable middle-ground position in French as well.

Avoiding subsequent reference by pronouns would, of course, help keep the discussion more gender-neutral both grammatically and sexually, but all concerned should try to make the effort to understand that subsequent references by pronouns and adjectives would be derived from the grammatic gender of the words “personne” and “individu” and not from the actual person’s or individual’s sex.

To the extent that feminine-form pronouns and adjectives referring to and describing “cette personne” and masculine-form pronouns/adjectives referring to and describing “cet individu” might be incorrectly confused (or purposely ignored as inadequate) as referring to the person’s/individual’s sex and not simply to the grammatic gender of these two gender-neutral words, perhaps systematically alternating between them during the course of a discussion could remove, or at least blur the gender labels.

So in your passage, I’d propose (and please [feel free to] excuse my French errors to the extent that they don’t render my translation incomprehensible):

Quand j’ai rencontré Bry Bitar, j’ai cru que cette personne avait 17
ans, mais en fait cet individu [si] charmant, cette personne [si]
grande n’a que 13 ans.

Et oui, Bry préfère se décrire avec les mots personne ou individu
plutôt qu’avec homme, femme ou n’importe quel mot qui attribue une
constitution ou « étiquette » génétique aux gens.

Frankly, when I see how the issue of avoiding civil status labels for over half of the population was addressed in ‘Francophonia’ (i.e., by defaulting to an existing title, Madame, as opposed to creating/resurrecting a status-neutral title as was done in English with Ms (not an abbreviation of Miss, btw) and in light of the hard-fought struggle to ADD gender-specific endings to words/professions that had previously defaulted invariably to one gender or the other, I am inclined to believe that this issue will likewise be addressed, not by neologisms, but by defaulting to existing words and grammar rules and theories (as my wife’s) already in place.

(Regardless, the status quo surely beats resorting to using the French equivalent of the pejorative “it” [“Ça”?], which far too many of my bigoted com‘patriots’ use to avoid giving credence and recognition to the singular “they” and value to the individuals represented thereby.)