Siren teaches an eclectic mix of Spells, consisting mostly of debuffs. She is the exclusive teacher of Sleep and Mute, and teaches all her Spells at a generous rate. She can also teach Fire at ×6, which can be a quick way to give non-natural spellcasters an attack Spell. Her Hope Song inflicts Mute on all enemies.
Siren offers the first Level up bonus to HP. This can be useful in the short term, though in the long run every Character will approach maximum HP even without bonuses.