Aquarius

Aquarius, "The Water Bearer" is a long-haired mermaid that can only be summoned into a body of water. She is selfish and dislikes Lucy, often attacking both Lucy and Lucy's enemies with a torrent of water.

Spoiler Warning: Click to View

The key of Aquarius has been destroyed by Lucy in order to summon the king of the spirits during the fight against the dark guild Tartaros. Meaning that she won't be able to meet Lucy again.

(Source: Wikipedia)