his or her real name
Personne ne connaît son vrai nom.
No one knows his real name.
C'est pas malin d'utiliser son vrai nom.
It's not smart to use your real name.
— Le Bon, la Brute et le Truand (1966)
Don't use his real name.
Je connais pas son vrai nom, mais par ici, on l'appelle Grands-Pas.
I don't know his real name, but around here, we call him Bigfoot.
— Le Seigneur des anneaux : La Communauté de l'anneau (2001)
What is his real name?
That's not his real name.
Je ne connais pas son vrai nom.
I don't know their real name.