From the end of "El Relato de un Náufrago" by Gabriel García Márquez.
Casi todos los días voy al cine. Y siempre acompañado. Pero el nombre de la acompañante es lo único que no puedo revelar, porque pertenece a la reserva del sumario.
The closest translation I could find is "It is the subject of the indictment." which doesn't make sense here because there is no trial. Can it mean "it's up to the discretion of the subject" or merely "because it's a secret"?
You are right. The expressions "reserva del sumario" or "secreto del sumario" mean, according to the Diccionario del español jurídico:
Declaración expresa en virtud de la cual las diligencias de investigación permanecen secretas hasta la apertura del juicio, pudiendo ser total o parcial.
Statement under which the investigation proceedings remain secret until the beginning of the trial, be it total or partial.
The expression can be translated as "secrecy of the investigation" and, as you say, implies a future trial. But you are reading a novel, and things like that are just literary resources, so they cannot be taken to the letter. What the character is saying is, as you have correctly predicted, that the name of his companion is something he prefers to keep in secret.
