Nymph is one of four basic Manas aside from Mana of Fire, Wind and Metal. She is vital for synthesizing water based items and most of healing items require the player to make a pact with her. She governs the element Water and despite her cute appearance, she is quite powerful.
Nymph; Child of Water, is Nell's first Blades Master, obtained after defeating her in a Mana Boss Battle. Nell's blade when in this form is staves. Nypmh is the second mana that can be "unlocked" in AI 3: GP. While using her as Blades Master, Nell's role is mainly change into healer, as most of her skill is have healing prowess when in this form, as well as buff support. Also, she has skill that will removing status negation (like ATK down, DEF down, etc).
When summoned by Iris in battle, Nymph unleash a very big ice block, dealing water elemental magic to one enemy.