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! (心のビートはもう止められないわ！ Kokoro no bīto wa mō tomerarenai wa!?)".
Role in the series
Before being brainwashed Siren was a kind cat. She always cared for Hummy despite her being naive to most things. However she could get very envious 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 and pendant broke and became Cure Beat.
As Ellen, she underwent a dramatic personality change. She has become more upbeat 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 practicing her introductory speech, showing just how passionate and committed she was to being accepted as a friend.