She has a kind and gentle personality, but due to her poor health she is usually unable to leave her castle, preferring to spend much of her time in her extensive gardens. She has a love of plants and flowers and appreciates the work her groundskeeper Emilio does, unaware of his true identity as the same demon she sealed away.
Sylphine's leadership role appears to be more ceremonial than anything, as the country continues to function even if she is captured early in the game. Her more important duty is to journey into the labyrinth and periodically reinforce the magic keeping the Overlord confined. In fact, the need to perform this ritual is the only thing keeping Euclid's supposed allies, such as Estelle's home kingdom of Zeidram, from simply conquering the weaker nation.
If Sylphine is confirmed as the heroine, she receives word that, due to her failure to keep the Overlord contained, Zeidram's army will be entering her kingdom to invade the Labyrinth and destroy him for good. Sylphine realizes that this is only a pretense, and Zeidram will most likely annex Euclid once the threat has been dealt with. Deirdre appears to her then, offering her assistance in defeating Emilio, which she reluctantly accepts.
Emilio and Sylphine's forces clash in the Labyrinth, but as he seems about to claim victory, Deirdre reveals her surprise attack. Both Vidal and Sassoone have been brainwashed, and shock Emilio by overpowering him and then draining the vitality from him. Her mission accomplished, Deirdre throws both Sylphine and Emilio in the dungeon, having never planned to keep her word.
Sylphine manages to escape from her cell and, making her decision, frees Emilio as well. She offers a bargain. She will give her body and power freely to him, helping him recover his body; in return, he must swear to protect Euclid from its enemies. Emilio agrees, and as they seal the agreement in her bedchamber that night, he notes that her freely-given power is very different from what he's accustomed to.
They eventually defeat Deirdre and recover Emilio's lost body; Sylphine tells Emilio that she'll hold him to his promise, and he says that fighting to protect something doesn't sound so bad. He proceeds to overwhelm Zeidram's military forces, driving them from Euclid's territory. Some time later, Sylphine has become a nun, telling a pair of children the story of "the demon lord and the princess," who to this day continue to watch over the kingdom from the shadows.