Phoebe Halliwell is one of main characters in TV-series Charmed, she was turned into a mermaid for first two episodes in season 5, called "A Witch's Tail". Phoebe Halliwell is portrayed by actress Alyssa Milano.
Phoebe Halliwell isn't a regular mermaid, she is actually a witch, but after a location spell, she was transformed into a mermaid. She hears the call of the sea and her heart starts to go cold. And Cole's plan to get her back doesn't help either.
When Necron comes for Mylie's soul, he finds Sea Hag vanquished and Mylie gone. Then Phoebe arrived as she had taken the seacave as her lair. Necron attacked and wounded Phoebe as she swam away. Phoebe came to ashore, and she was captured by fishermen. The fishermen however were attacked by Necron, and Phoebe finally called her whitelighter Leo and he orbed her back to the Halliwell mansion. Necron then kidnapped Piper and brought her to the Sea Hag's seacave. Phoebe swam back to the cave and Necron started to train her mermaid immortality, until Paige arrived and orbed the Auger Shell away from Phoebe. The Charmed Ones then vanquished Necron, but Phoebe went back to the sea. Later Cole teleported Phoebe to himself and convinced Phoebe to admit her own feelings, turning her back to a human.