Edea’s part in this story is perhaps the most important one in term of negative and positive affect. She truly is both the Matron and the Sorceress, giving her character a level of irony [obvious irony] that ties into the literary factors of this wonderful game’s story.
The start [and the end] of it all is the day her powers are passed to her. Time compression at its end, an injured Ultimecia wanders time finding a home for her powers to that she can die in peace while Squall is still trying to get out of the blur and back home. They both end up in Edea’s yard at the orphanage.
She speaks to Squall about the future and the sorceress before Ultimecia passes her powers. And, from that point forward the time paradox begins [or continues] itself with her idea of SeeD and her powers from Ultimecia.
In the ‘present’ day of the game, she probably causes more problems than she is helpful aside from her creation. Her being the powerful sorceress of the time, it’s no wonder she is reeking havoc over the world and slowly rising to political domination. In their final battle against her in Galabadia Garden, she passes her powers to Rinoa, a key point in the story that controls many of the incidences in the second half of the story.
Of course, once Edea returns to herself she does help the group out by leading them to Esthar and given them a lot of information, but her role in the story dissipates. She leaves the foreground, but her aspect of the story is still of great importance. As the perpetual paradox of the game goes on, she will remain the beginning and the end of it; the mother of fate.