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.
When Hummy gets kidnapped by Mephisto and gets tortured Sirien turns against him and saves Hummy. When Mephisto starts to target Hummy yet again, Siren calls out and transforms into Cure Beat. Siren, however, is unable to take everything in and runs off. Soon after that she finds out that she can no longer transform into a cat or change her disguise.
As Ellen, Siren continues to reject Hibiki and Kanade's offer of friendship as she is still unable to forgive herself. 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 a Negatone, Ellen overcomes her worries and officially joins the Pretty Cures.
Hibiki and Kanade call Siren by her human form name 'Ellen', but Hummy and Minor Land inhabitants still call her by her real name.