who is budding young opera star Erik's student and Raoul's love interest?

Christine Daae is first mentioned in the prologue.

The events do not date more than thirty years back; and it would not be difficult to find at the present day, in the foyer of the ballet, old men of the highest respectability, men upon whose word one could absolutely rely, who would remember as though they happened yesterday the mysterious and dramatic conditions that attended the kidnapping of Christine Daae, the disappearance of the Vicomte de Chagny and the death of his elder brother, Count Philippe, whose body was found on the bank of the lake that exists in the lower cellars of the Opera on the Rue-Scribe side.