Following on the heels of my pixie class, here's a writeup for nixies as PCs. This one was actually done a while ago, after one of the PCs in Doom & Tea Parties got transformed by a chaos critter. She's going to be stuck as nixie for a while, but so far she doesn't have any serious complaints.
Nixies (and nox, as the males are called) are aquatic fey who dwell underground. They rarely adventure, but as the guardians of the pathways between Water and the other realms, some are called to journey far from their subterranean communities. They resemble elves, being slight and short (both sexes tend not to grow taller than 5.5') with pointed ears, and almond shaped eyes. However, they also have blueish tint to their skin and green hair.
The prime requisites for nixies are are Intelligence and Charisma. If an nixie has a score of 13 or greater in both Intelligence and Charisma, the character will gain a 5% bonus on earned experience points. If the nixies Intelligence is 13 or greater and her Charisma is is 16 or greater, she will earn a 10% bonus on earned experience.
RESTRICTIONS: Nixies use six-sided dice (d6) to determine their hit points. They may advance to a maximum of 10th level of experience. They may use any shields or weapons, but suffer a -1 to saving throws when wearing ferrous (iron or steel) armour. They may wear armour of other metals (such as bronze, orichalcum, or adamantium) without suffering this penalty. They cast magic user spells as well. They roll saving throws and fight as elves. A character must have an intelligence of 9 or greater to be a nixie.
SPECIAL ABILITIES: Nixies are able to see in low-light conditions as if it were early evening illumination. Nixies can speak with all aquatic creatures and may summon fish to perform simple tasks. They may cast a water breathing spell that lasts for one full day. A group of 10 nixies can cast a powerful charm spell on humanoid creatures. They perform underwater as if under the influence of free movement magics, being completely unhindered by the water around them.
Art by John William Waterhouse, because when it comes to nixies, nymphs, sirens, and sorceresses, he is the man.