Water Nymphs are water spirits from traditional folklore, in this case depicted as having moving bodies of water for hair and wearing swimsuits. The Nymphs are known for their lack of being able to tell a good joke without messing it up.
The Nymphs make their first appearance in the episode "Power Animal." They're seen playing in the pond near the Tree Fort telling terrible jokes to each other. They later attend a party in the Cloud Kingdom.
The Nymphs return together in "Thanks for the Crabapples, Giuseppe," with a new Nymph, who was first seen texting on her pink phone and later demonstrated to be very active in making decisions for the other Nymphs and giving out orders. When Abracadaniel failed in luring them into joining their trip, Ron James enticed them by putting on Eye Twinkles. When the bus has sunk in a lake, the girls attempted to help. However, when they failed to lift the bus, they flee selfishly, leaving the others behind.
In the "Frog Season: Summer" short, three water nymphs appear on donut floats while going down a river. One of the nymphs invite Finn and Jake to go into the water but are turned down. Later in the short, when Jake forcefully puts the crown on the frog, the same water nymph states a saying.
The first two Nymphs both have green-tinted skin and hair that looks like rushing water. Denise has long hair and wears a pink bikini. She is also skinnier and taller than her friend. She wears multiple water bracelets on her arms and leg, and has a gold bellybutton piercing. She also has pink lipstick while her companion has none. She has white eyes.
The other Nymph has short hair, similar to Princess Bubblegum's at the end of the episode "Death in Bloom," and a blue bikini. She is slightly curvier, shorter and has bluer skin than her companion. She has no belly button piercing. She has black eyes.
The third Nymph, unlike the others, has blue skin. She has long hair tied in a ponytail and wears a straw-colored bikini with white stripes.
- Adam Muto stated that their looks were based on normal girls.
- Denise (the taller nymph) is voiced by Hynden Walch, the same person as Princess Bubblegum. The shorter nymph is voiced by Niki Yang, the same person as Lady Rainicorn and BMO. The third nymph is voiced by Grey DeLisle, the same person as Ice Queen, Breakfast Princess, and Peanut Princess.
- The forgotten joke is also part of the stand up routine (at 3:03) of comedian Melinda Hill who does the voice of Doctor Princess. She claims it is how her mother tells a joke.
- The Nymphs appear as save points as well as in the Cloud Kingdom in the video game Adventure Time: Hey Ice King! Why'd you steal our garbage?!!
- According to the comics, Ice Queen was originally a Water Nymph.
- The long-haired nymph appears again in "Wake Up," where her name is revealed to be Denise.
Denise: I've got a joke. What did the mermaid see a movie?