Merfolk

As shown in the short story, "For King, Country, and Sea Men," the merfolk were an undersea dwelling people who thrived in the warm tropical waters of both Earth and eventually Aiedrial. They are known for their graceful dances, love of theatre, and unusual appearances. However, they are made most famous by the Hans Christian Anderson story that was inspired by a real political event in the long-lost kingdom of the Nica Isles, known now as "The Little Mermaid." 
The merfolk are a loving people, though they are often distrusting of humanoids on the surface who make livings off of fishing. They tend to be curious as they like to watch ships sailing on the water and explore docks. They are most commonly found in the reefs and warm waters of the tropics, never daring to venture too far into the depths since the water pressure past a certain depth can be dangerous. 
They most commonly have a humanoid appearance with the exception of their lack of hair and webbed fingers and toes to help them swim. They also have legs rather than a single tail, despite what some stories and rumors would have one believe. Their skin tones will generally have very little color, though if there is color it will be in tones of green or blue. It is more common to see a merperson with nearly transparent skin, which is often why land-dwelling humanoids are disturbed by their appearance. They have muscular arms and legs because of their constant swimming, though when they go on land they are quickly fatigued by walking, especially uphill. 
It is possible for the merfolk to go on land as they can breathe both water and air, though they usually prefer not to since being on land is exhausting to them and can be potentially dangerous. Their limited exposure to direct sunlight makes them especially susceptible to severe sunburns and overheating, so they must wear lightweight clothes that cover their entire bodies to protect them from the sun. They must also be careful about keeping their skin moisturized while on land since they are also susceptible to dehydration and drying out. Though they don't need to drink water like other humanoids do, they will not survive longer than 48 hours without at least soaking in a bathtub to keep their skin healthy and their gills functional. 
It is difficult for land-dwellers to visit the merfolk without the use of clunky machinery, which often makes the merfolk nervous. They are hesitant to trust large machines in their waters, especially those that could potentially polute them, but are still willing to meet with whoever may come as long as it is in peace. When entertaining guests, they are likely to show off their best water dancers who will act out stories of love, war, and tragedy. The dances have been compared to ballet, though the lack of music has often made people disagree. 
One of the most famous merfolk was Princess Aerwyna from the Before Treaty Era, who nobly accepted a marriage to the human prince of the Nica Isles in order to stop a long-standing war between them and the merfolk. She faced several discomforts in the human world, but did find love with the prince and was happily married for the rest of her life, despite the fact that they could not produce an heir together and instead had to adopt one heir each in the respective kingdoms, (there are even rumors that the heir chosen for the Nica Isles was actually the illegitmate son of the prince and that the arrangement was partly to cover up a scandal, but this has not been confirmed). Historical records in Aiedrial say that even with the peace between the humans and merfolk set, civilians of the Nica Isles were hesitant to accept the marriage between the prince and princess as legitimate until a pamplet was published detailing the story about how they met and fell in love. Needless to say, Anderson took several creative liberties with his work on "The Little Mermaid," to the point where it is barely recognizable as Aiedrian history. However, it has grown to be beloved by the humans of Earth.

Naming Traditions

Unisex names

Merfolk tend to use names that have sounds similar to the sounds of water and wind, generally giving them very soft, flowing sounding names or brash, hard names like the ocean during a storm. The names are not gendered, but try to appeal more to the person's personality. For this reason, it is common for a merperson to not officially be named until they are a few years old. Until then, they are given childish nicknames such as "Kelp" or "Drift", which may remain with them as terms of endearment from their family members even after they are grown.

Family names

Considering that merfolk families tend to be very big, family names are generally shortened to the name of the kingdom they live in.

Culture

Major language groups and dialects

The merfolk are all educated in the common languages of the land-dwellers, but often speak to each other in a form of sign-language since sound does not carry well underwater.

Ideals

Beauty Ideals

Grace is upheld as the greatest trait of beauty, though smooth skin and soft features are also covetted by both genders. Their clothing will often be lightweight and flowing to add to the graceful appearance of their movements.

Courtship Ideals

Polyamorous relationships are not only common with the merfolk, they are expected. Merfolk tend to be very affectionate with their friends and partners, so physical romantic affection between multiple people is normal. Jealousy is often seen as a selfish trait, particularly because partners were usually linked through mutual friendships. Trust is the most sought-after trait in said relationships, so much so that if one person in the relationship breaks that trust, they will usually be left behind by the group and unwelcomed into other groups.

Relationship Ideals

Commitment is made between two merfolk when they choose to have children, as kingdoms are usually so small that they must limit themselves to having one partner to have children. Romantic affection may continue with others, and other merfolk in group relationships with the parents may even be considered part of the family and may help raise the children, but the decision to choose one person to share a bond with through the creation of a child that is made from pieces of the both of you is seen as the strongest display of love. If a person breaks that bond by having children with another, they are shamed not only for breaking that trust, but for breaking the rules of breeding in the kingdom.