Ondines were mermaid-like creatures from Japanese Folklore, that Bobby encountered in Bobby Singer's Guide to Hunting.
Close to mermaids, the Ondine were creatures from the sea who took men, mainly sailors, away to feed. Live in close family units, the females do most of the hunting, while men guard the children and home. Ondines have monogamous relationships, and thus they detest men who have cheating hearts, preferring to feed on these kind of men. Their family bonds are so strong that the Ondine "Keiko" refused to kill Bobby and even befriended him due to sensing the great loss Bobby was feeling over the death of his wife. The population is mainly extinct due to starvation and loss of habitat.
During Bobby Singer's time on a Japanese Merchant Ship the "Nishigo Maru", the engines were destroyed and crew members were mysteriously vanishing one at a time. Bobby originally suspected the killer might be an "Ikuchi" a giant sea monster from Japanese Folklore however he soon discovered an old woman he had befriended, was an Ondine. She was the matriarch of an Ondine clan and was killing the sailors to feed her family.
- Shapeshifting: Ondine can take the shape of a human but their natural form is covered in scales, with the tail of a fish and teeth like a shark. Their true form can only be perceived by humans, while they are intoxicated.
- Underwater Breathing: They live and survive underwater similar to fish.
- Super Strength: Ondine possess strength superior to humans.
- Invulnerability: Ondine can not be killed by conventional methods, such as stab wounds or gunshots. Bobby shot Keiko in the back twice, but it had no effect.
- Infection: Ondine can transform humans into their species through bite, during this a warm blue light projects from their mouth.
- Empathy: They posses abilities to read human emotions and sense how they feel.
Bobby suspected that these methods might kill them but he was never able to test them.