Siren (セイレーン Seirēn) is Hummy's childhood friend and singing mentor, both born and raised in Major Land. For years before series start, she was Major Land's Fairy of Songs, and sung the Melody of Happiness every year. However, for the current year, Hummy was chosen to sing it instead. She, allegedly feeling betrayed, traveled to Minor Land to ask to become Mephisto's subordinate. But in truth, she was brainwashed as a tool so that he could complete the Melody of Sadness.

She had the ability to shape-shift into any form she wished, and often alternated between her true feline form and her alter ego Kurokawa Ellen (黑川 エレン Kurokawa Eren), using the charm on her necklace. She could also summon Negatones from the scattered musical notes using the pendant. However, after becoming Pretty Cure, she has lost her pendant and thus both of these abilities.

Ellen's alter ego is Cure Beat (キュアビート Kyua Bīto). She uses the light blue Fairy Tone Lary to transform. Her catchphrase is "Can't stop my heart's beat!". Is the love interest of Masamune Ouji.

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