Siren

Siren is a friend of Hummy, also born in Major land. Last year she sung the Major Land's Melody of Happiness. But this year Aphrodite chose Hummy to sing it instead of her. She then felt betrayed and went to Minor Land to ask Mephisto if she could be his servant. But in truth, she was brainwashed as a tool so he could get the Legendary Score. She has an ability to Summon Negatones from the scattered notes. Besides being a cat like Hummy she also has a human form named Ellen Kurokawa (黑川エレン). Her cure alter ego is Cure Beat (キュアビート). She uses the light blue Fairy Tone Lary to transform.

Betrayal
Siren, feeling betrayed by Aphrodite because she chose Hummy to sing the Melody of Happiness, goes to Mephisto of Minor Land pledging her loyalty. On the day that Hummy was due to debut as the new singer of the "Melody of Happiness", Mephisto attacks stealing the Legendary Score and rewriting the notes to create the "Melody of Sorrow". He then Called forth Siren to sing the new melody. Aphrodite was shocked over her betrayal as she began to sing causing the crowd to start crying and feeling sad. Aphrodite stops the song and spreads the notes to the land where warriors are born. Mephisto sends Siren and the Trio the Minor after them.

First Encounter with Pretty Cures
Siren, in the guise of Ellen, meets Hibiki first and is eager to take the G Clef sealed in her heart. Hibiki runs outside only to see Kanade holding a record the two girls used to listen to together. Siren sees Kanade has the same G Clef as Hibiki and orders the Trio the Minor to take Kanade's while she takes Hibiki's. Before they even reached the G Clefs, the emblems suddenly shone, throwing back Siren and Trio the Minor. As she regains her footing, she sees one of the notes from the Legendary Score on the record and quickly transforms it into a Negatone which calls forth a monster to attack the city and spread the sorrow. Hibiki and Kanade were angered by this, and the emblems in their hearts came out and transformed into Cure Modules, which allowed the two girls to transform into Pretty Cures.

Betrayed Again
During episode 10, Siren was injured and was about to get hit by a car until Ouji saved her. In his presence, she cannot reveal her true nature to him but seemed to have developed a bond with Ouji. The next day, Siren hesitates due to Ouji's presence and one of her stooges, Bassdrum, transforms the gifts Hibiki and Kanade received into Negatones which they defeat. Afterwords, Bassdrum denounces Siren as their leader. Later, Mephisto declares Bassdrum as the new leader due to Siren's sudden change.

The Big Lie and Being Brainwashed
During Episode 13, Siren sees Hummy telling Hibiki and Kanade about the times when they were still friends. Hummy is able to reach her heart and Siren begins to cry. Then Falsetto and Baritone appear, in disguise, and inform the Cures that there is a "Negatone" wreaking havoc in the town. Once Hummy and the Cures are out of the way, Siren sees one of the old contestants who tells her that Hummy told the judges for the audition that Siren had given her a fake score. Mephisto appears and tells her the same thing, but Siren still believes in Hummy. She then tried to oppose him but Mephisto brainwashed her again, making sure he erased her memories of being with Hummy and becoming a tool to serve him.

Rebirth and Regrets
After Siren obtains the Fairy Tones and the scattered notes, Mephisto invades Kanon Town and sets up a stage to perform the Melody of Sorrow. Aphrodite speaks with Hibiki and Kanade, asking them to believe in Hummy. However, Hummy is kidnapped by Mephisto and is given headphones to amplify the Melody of Sorrow. As Hummy nonetheless expresses her words of friendship to Siren and sings, Siren remembers the good times she spent with her and is moved to tears. As Mephisto tortures Hummy with the Melody of Sorrow, Sirien turns against him and tries to free the Notes, but Mephisto turns them into a Negatone. Hummy manages to free herself and the Fairy Tones, allowing the girls to transform. As Mephisto targets Hummy yet again, Siren calls out and transforms into the Pretty Cure of Miracles: Cure Beat, whilst Hibiki and Kanade defeat the Negatone. Siren, however, is unable to take everything in and runs off.

Everyone becomes curious as to how Siren became a Pretty Cure, especially Siren herself, who finds she can no longer transform into a cat or change her disguise. Left confused as to where to go, she winds up at the church where Mr. Otokichi gives her comfort. Hummy finds her, but she feels she doesn't deserve to be forgiven so easily. As Hummy follows her around, with Hibiki and Kanade also offering their friendship, they are approached by the Minor Trio who create a Tree Negatone. As the Pretty Cure run into trouble and Hummy is captured, Siren stands to protect and produces a Cure Module, allowing her to transform into the Pretty Cure, Cure Beat. Using her new abilities, she rescues Hummy and defeats the Negatone. After the battle though, Siren is still unable to forgive herself and runs off again.

Forgiveness and Joining the Team
Now known as Ellen, Siren continues to reject Hibiki and Kanade's offer of friendship as she is still unable to forgive herself. She encounters a boy named Mamoru who is hiding from his father. He explains that he ran off with his bag after hearing he was going to work abroad for a year. As Hibiki and Kanade have conflicting ideals to Ellen, they go to search for the father whilst Ellen leaves with Mamoru. Mamoru soon discovers a stuffed animal inside the bag that his father made him and decides he wants to return, helping Ellen realise she's not alone as well. However, just as father and son are reunited, Bassdrum spots a Note inside the doll and creates a Negatone from it. As Bassdrum tries to waver Ellen's feelings, Hummy helps her build up the courage to join in the fight as a Pretty Cure. After defeating the Negatone, Ellen overcomes her worries and officially joins the Pretty Cures.

Ellen Kurokawa
Ellen Kurokawa is Siren's human form, which she uses to disguise herself at some points in the story to lower the Harmony Power of the Pretty Cures and to make sure she can get the Scattered Notes. In this form however, she has shown to be a strong fighter and is able to materialize notes and use them as projectiles.

Cure Beat
" Playing the Soulful Tune, Cure Beat! "  爪弾く魂の調！キュアビート！ "Tsumabiku wa Tamashī no Shirabe, Kyua Bīto!" Cure Beat (キュアビート) is the Pretty Cure alter ego of Siren/Ellen, which debuted briefly in Episode 21 after she betrayed Mephisto and saved Hummy, allowing her to transform for the first time but without a Cure Module. She then later transformed fully in Episode 22, using her Cure Module along with her Fairy Tone, Lary. When transformed, her normally dark purple hair changes to an intense purple color and gains a ponytail. Cure Beat wields a Guitar-like Belltier called the Love Guitar Rod and she can use a purification attack called the Heartful Beat Rock and several weaker attacks like the Beat Barrier and Beat Sonic.

Relationships

 * Hummy - Siren was a friend of Hummy, but after Aphrodite asked Hummy to sing the song instead of her, Siren felt betrayed and became Mephisto's right hand. She tends to yell at Hummy as to try to argue but Hummy is just so optimistic it doesn't really matter. After she found out the truth, she then apologized to her until she was brainwashed by Mephisto once again. In episode 21 Siren seems to shake this off when she saves Hummy from Mephisto and becomes Cure Beat.

Ethymology

 * Siren - Siren is a reference to Greek myth, specifically the Odyssey where a group of beautiful witches called the "Sirens" would sing to lure sailors to their deaths by making them crash their boat into the rocky shores of their island.
 * Kurokawa (黑川) - The name Kurokawa has two terms. Kuro means black in Japanese, which fits Siren's theme color while Kawa means river. Her name can be roughly translated as Black River.

Trivia

 * Siren is the first mascot-like character to be a villain. She is also the second mascot-like character to become a Pretty Cure, the other being Milk, who obtained status equal to that of a Pretty Cure.
 * Siren is the second villain who is been rejected by a villainous group due to her weakness, the other being Higashi Setsuna of Fresh Pretty Cure!. Siren is the second villain to become a Cure, the other being Setsuna again.
 * More strangely, Cure Beat shares the same colors as Cure Berry from Fresh Pretty Cure!.
 * Siren also has a human form like the mascots from Yes! Pretty Cure 5, Yes! Pretty Cure 5 GoGo!, and Coupe from Heartcatch Pretty Cure. She first introduces herself in this form to Hibiki as Ellen.
 * Although her main form is Ellen, Siren also seems to be able to disguise herself as any human, as she does this to try to fool the Cures.
 * Siren is the only mascot character besides Hummy to be a cat.
 * Siren as Ellen has used her abilities to decrease Hibiki and Kanade's Harmony Power by disguising herself as a transfer student named Houjou Sakura.
 * Cure Beat's transformation sequence is indeed a big tribute to Sailor Moon. Coincidentally, Hummy's voice actor did Usagi Tsukino herself.
 * Siren seems to have lost her ability to transform into other people including her cat form after she first became Cure Beat
 * Even thought Siren lost her ability to transform into other people including her cat form and gain the abiliity to transform into Cure Beat, her true form is still a cat.
 * It is revieled in episode 24 that Ellen is afraid of crabs, as Hibiki and Kanade reffer to it as cute.