In Legend, the first of a trilogy, both of the main characters are definitely famous—though for very different reasons. Day is the Republic's legendary criminal; he's the one they're trying to hunt down because he keeps stirring up trouble and disappearing before they can catch him. June on the other hand is one of the elite. She is the Republic's prodigy child—the one who got a perfect 1500 score on her Trial. She's also slated to graduate from college very early and join the military.
The book is all about the meeting of these two very different legends, so it would make sense that the title would reflect that. And perhaps the choice to make the word singular rather than plural is a deliberate one. After all, Day and June find out that they may be from vastly different backgrounds, but deep down inside they have a whole lot in common.
It makes sense, too, that the next book in the series is called Prodigy—since it's about the two prodigies of the Republic. The final book is called Champion, so we just have to hope that this means that Day and June will win out at the end.