Cyrene was born in Thessaly, the northern part of Greece, to Hypseus, the King of the Lapiths, and Khildanope, a naiad. Since Hypseus' grandfather was Oceanus, and his father, Peneus, was a river god, Cyrene wasn't a full mortal herself.
Guardian of Thessaly's Animals
Cyrene grew up dreaming of war and conquest, and wanted to be a great warrior like her father, who put her in charge of guarding the cattle and the sheep. She took to this job quickly, slaying any predator (mortal and supernatural alike) who posed a threat to the herds and flocks. She proved to be such a deadly efficient guardian that all the cattle and the sheep completely entrusted her with their safety, and she had no qualms with driving them to dangerous places so she could hunt and kill more dangerous monsters. Her extraordinary hunting skills caught the attention of Artemis, who gave her two fine hunting dogs as gifts and tried to recruit her to join her Hunters, but Cyrene respectfully declined the goddess as she still wished to be married one day.
With her new hunting dogs, Cyrene became a deadlier warrior, to the extent where all the local predators steered clear of her and the animals under her watch. One day, she spotted a lion stalking her flock and, despite her lack of weapons at that point in time, she still confronted the lion and managed to strangle it to death. Unknown to Cyrene, Apollo, who had heard of her from Artemis, had been watching her fight with the lion and desired to take her as his lover. He appeared to Cyrene in all his majesty and courted her with praises of her virtues, even with the promise of a kingdom of her own to rule if she would go with him. Intrigued by Apollo's handsomeness and sweet words, she agreed to go with him and took her faithful hunting dogs with them as well.
Queen of a Colony
Apollo took Cyrene to the northern coast of Africa (known in the modern times as Libya), where they enjoyed an intense and passionate love affair. Thanks to her lover's Oracles spreading the word of a new queen, an entire colony of Greeks came to thrive in her new home. They built a city that was named after Cyrene and dedicated their most important temple to Apollo. Her city became the first and most important Greek colony in Africa, and - though she was unaware of it then - it was to survive through most of the Roman Empire.
Apollo and Cyrene also had two sons: Aristaeus (who later became a minor deity of beekeeping and cheese-making) and Idmon (a seer who still chose to die a hero's tragic death despite his having foreseen it). In spite of all this, Cyrene grew discontented with her life - her hunting dogs passed away, her children grew up, and Apollo's visits became increasingly seldom as it was custom for gods to tire of their mortal lovers in time. Hence, she decided to go back to Thessaly one more time to visit her childhood friends and see her father.
Return to Thessaly
When she finally reached Thessaly, Cyrene learned that things had completely and irrevocably changed from what she had remembered: her father had passed away and the new King of the Lapiths didn't want anything to do with her. Her friends had either married, didn't even recognize her, or had died from the harsh life the Lapiths led.
As she roamed the old paths where she used to herd sheep, Cyrene encountered Ares himself and engaged him in a battle that unexpectedly concluded with their becoming passionate lovers. They stayed together for many months and had a son, Diomedes, who became the King of Thrace.
Rebirth as a Naiad
Cyrene eventually returned to her city on the African coast and found Apollo waiting for her at the exact same spot where they had first landed many years ago. Despite her romance with Ares, Apollo assured her that she owed him no explanations, and acknowledged his faults of being an increasingly inattentive lover to her. However, he still observed that their romance was at an end, to which Cyrene was relieved as she was ready for a peaceful and quiet life.
After assuring Apollo that her city was where she belonged, Apollo transformed Cyrene into the patron naiad of her city, promising that her new state of existence guaranteed her exceptional longevity, peace, and good health. With that, Apollo's and Cyrene's romance came to an official amiable end.
As Apollo had promised, Cyrene the Naiad lived a very long life, and she eventually left her Greek colony to live full-time with the other naiads in the nearby rivers. However, she would occasionally rise to offer advice to her friends and family - once when her son, Aristaios, lost his sacred bees, Cyrene helped him find them again.
In Percy Jackson's Greek Heroes, Cyrene was described to have long dark hair, a face that was beautiful and serene even in the midst of combat, and graceful limbs that were the color of polished bronze in the sunlight. When Apollo saw Cyrene for the first time, he noted that she reminded him of a Goddess of War. The poet Hesiod also once described her to be as beautiful as the Charites.
- Battle Prowess: Cyrene was a formidable and skillful warrior, being an expert in the spear and the sword. Her "war against wild animals" resulted in the sheep and cow under her watch having a complete trust in her to preserve their safety against all threats. She was even able to hold her own against Ares himself in battle.
- Hunting: Cyrene was an expert in hunting and killing vicious predators, even those who were much larger than her. She was such an accomplished huntress that the animals under her watch never worried about nearby predators due to their utter trust in her, with the possibility of her having "killed every predator within a fifty-mile radius". In fact, her hunting skills impressed Artemis herself, and the goddess tried to recruit her in the Hunters. Though Cyrene declined, Artemis still retained a favorable opinion of her. As an expert huntress, she was also particularly skilled in:
- Tracking: Cyrene was an expert in seeking out any and all potential threats to the animals that were under her watch - a skill she developed from her constant patrolling of the hills, the wilderness, and other dangerous places. Even after she became a naiad, she was shown to have retained this skill: she once helped her son Aristaios to find his sacred bees when he lost them.
- Archery: Artemis described Cyrene as being "amazing with the bow", which shows that she was an expert archer.
- Superhuman Strength (possibly): As a legacy, it is possible that Cyrene possessed superhuman strength - she fought and killed a large male lion without any weapons, picking it up by its back legs, tossing it into a boulder, and finally strangling it to death.
According to Apollo, due to her great-grandfather being Oceanus and her grandfather a River god, Cyrene had always been part water spirit, though she never tapped into the unique watery abilities that that heritage gave her. After her transformation into a full naiad, Cyrene came to possess supernatural powers associated with water, and it is highly likely that those abilities from her heritage were enhanced to their full potential as well. As a naiad, examples of supernatural powers Cyrene possessed were:
- Longevity: Cyrene was said to live a very long life after becoming a naiad, though it was never specifically stated or confirmed if she had become truly immortal.
- Clear Sight: After becoming a naiad, Cyrene was able to always and perfectly see through the Mist - when the transformation was complete, she saw the nature spirits of the sky and the trees, and could hear the music of the streams.
- Hydrokinesis: As a descendant of Oceanus, and later a naiad in her own right, Cyrene had considerable control and manipulation over water, especially the rivers of her city.