Main Character?
"They should know the way we, or at least “I”, should take. So, there is no need for you to come."While Dart is considered the hero of the game, the game is not necessarily only about him. It is quite possible the game intended to have two main characters, Dart and Rose.
There are many hints throughout the game that imply Rose is just as central to the story as Dart is. Dart is the first party member, never leaves the party and is the natural leader of the group. Rose joins much later, leaves the party for a while, and acts as a guide for Dart. At first glance it may seen she is simply a dark anti-hero to contrast Dart. There is much more to her than just a character foil.
Image Song
"Do you still remember All the time that has gone by"
Rose was the first character to be shown on screen. She introduced Feyrbrand before jumping off screen into the forest to face him. Her first appearance set up Rose's persona for the entire game. She was mysterious woman who knew far too much. Before you start the game, an introduction cinematic will play. The song played during this cinematic, "If You Still Believe", doesn't seem to have much to do with Dart as a main character. It does however reflect Rose’s situation. The song frequently refers to long passages of time, last goodbyes, broken promises of never being apart and waiting for a loved one.
One could argue this could also apply to Shana and Dart however it's harder to match their relationship to the song. Rose and Zieg's relationship readily encompasses the lyrics.
Responsibility
Rose was directly responsible for many of the past events in the game. She was a leading member of the Dragon Campaign. She was the Black Monster. She saved the world at least 100 times and shaped Dart into the man he is. Her role in the game isn't delegated to just past events. The plot of the game, stopping the destruction of the world, was Rose's mission. She started on that journey 11,000 years ago and game only showed the last year of that journey. Dart and his friends got caught up in Rose's mission and helped her finish what she started. In this sense, Rose was the main character as much as Dart was. They didn't take over her responsibility, instead they helped her complete it.
Screen Time
Rose had a very large amount of screen time compared to the other party members, excluding Dart. Only Shana came close to a similar amount. In some of the cutscenes or cinematics Rose appeared in, she was given equal billing to Dart. These include Dart's awakening as the Red-Eye Dragoon and the first encounter with Mappi. In others such as the first Virage encounter, Rose was a predominate character. She was the only character to receive two cinematics featuring her alone, although only one features her as a known character.
Her prominence throughout the story would seem to support the idea that she could be an equal main character to Dart, representing the part while Dart represents the future.
Press Kit
The French Press Kit Disc contains a text file with a brief story summary.
As the story unravels, players get caught up an incredible world of sorcery, swords and dragons. There are nine playable characters to choose from, with the three main characters; Dart, Rose and Shana each playing a very important and independent role.
This information supports the theory that Rose is also the main character and actually infers that Shana is also a main character bringing the total to three.