Siren is a female Cat-like "Fairy of Songs" and Mephisto's assistant. Although she was born in Major Land and is friends with Hummy, she became jealous when her title of the annual singer of the "Melody of Happiness" was given to Hummy instead and asked to be Mephisto's right-hand. As Siren, she has powerful abilities and can create Negatones using her dark powers. She can also use the powers of her necklace to take the form of any person she wishes. In her normal human form, known as Ellen Kurokawa, she is a strong fighter and can materialize notes to use as projectiles.
During Episode 21, her potential of Pretty Cure was awakened as she wanted to protect Hummy, who is captured by the Minor Trio. This caused her necklace to shatter, trapping her in human form of Ellen Kurokawa and causing her to lose her power of transformation. After joining up with the Cures, she begins living with the Shirabes and attends school with Hibiki and Kanade. As Cure Beat, her dark purple hair turns bright purple and gains a side ponytail. Her theme color is blue, and her Fairy Tone is Lary. Her main weapon is the Love Guitar Rod, which uses the Fairy Tone, Sory. She is unique to the Pretty Cure franchise as being a former mascot and antagonist that later became a Pretty Cure.
As a cat, Siren has smooth dark purple fur with lighter purple fur around her neck. She has a pink heart-like marking on her forehead and her back and she has several golden bands tied around her. Like Hummy, she has similar music note markings around her eyes.
As Ellen, she has mid-length dark purple hair, acessorized with a twisted golden band tied around one strand. She has sharp yellow eyes and an angular face. She wears a basic white shirt with a sleeveless, black jacket, a yellow choker, dark gray bracelets on each wrist, a short black skirt, long stockings and small boots.
Once she switched sides, Ellen replaced her twisted golden band with a yellow/golden scrunchie, which ties her hair in a side ponytail on the left side while letting the rest of it down. She also now wears a long-sleeved light purple shirt with a pattern of pink hearts on the front over a long-sleeved white shirt, a short black waistcoat with a pattern of pink hearts on the left, a short pink heart-shaped pendant, black bracelets on each wrist, a lavender ruffled skirt with a black belt, black stockings and short sand-colored boots.
As Cure Beat, her hair becomes wilder and longer, and is tied to the side with a feathered bow. Her costume is slightly more elaborate than the previous two precures, featuring many more frills. Her boots are knee-length and her gloves are wrist-length and fingerless.
Before being brainwashed Siren was a kind and empathetic cat. She always cared for Hummy despite her being naive to most things. However, she could get very envious and acquisitive of other people's achievements, such as when Aphrodite chose Hummy over her to sing the Melody of Happiness. After being hired by Mephisto she was cold, calculating, lacked sympathy and unwilling to open up to people. But there was still good in her, when Hummy managed to convince her that she was blindly following Mephisto and was being used. Her earphones broke and became Cure Beat.
As Ellen, she underwent a dramatic personality change. She has become more upbeat, sanguine, optimistic, and cheerful. She was also shown to be much kinder, but with this new-found kindness also came a streak of naivete, such as being easily offended or being prone to childish fears. After a chat with Hibiki and Kanade she became excited at the prospect of starting school. So much so, she spent the whole night practising her introductory speech, showing just how passionate and committed she was to being accepted as a friend.
- Siren is among one of the many the villains of the franchise to eventually join the side of the protagonists.
- Siren is the second villain to become a Cure, the other being Setsuna.
- Though she left Minor Land since Episode 21, Siren got the most number of known summons of Negatones, 15 summons in total. However, Bassdrum got two double summons, so they share the same number of summoned Negatones. Though, the Negatone in the beginning of Episode 5 could be probably summoned by her, increasing her number of summoned Negatones to 16.