Ellen Kurokawa originated as Siren, a feline-like Fairy of Songs from Major Land tasked with annually performing the Melody of Happiness. A deep bond with her childhood mentee Hummy fractured when Aphrodite chose Hummy over Siren for a coveted singing role. Feeling betrayed, Siren defected to Minor Land, pledging loyalty to Mephisto. She underwent brainwashing to serve his goal of spreading sorrow. As Mephisto's subordinate, she manipulated musical notes into Negatones using a pendant on her collar and could shapeshift, frequently adopting the human guise of dark-haired girl Ellen Kurokawa to infiltrate the human world. In this role, she operated with cold, calculating precision, displaying minimal empathy and emotional isolation. Her initial encounters with Cure Melody and Cure Rhythm involved attempts to steal their heart's G Clefs and creating Negatones to attack Kanon Town. Her resolve faltered after Ouji Masamune rescued her from a car accident, sparking unspoken affection. This hesitation caused Mephisto to demote her, replacing her with Bassdrum. A pivotal moment arose when Hummy's pleas briefly disrupted her brainwashing, but Mephisto reinforced control by erasing her memories of their friendship. Witnessing Hummy's enduring loyalty during Mephisto's Melody of Sadness performance triggered Siren's recollection of their past bond, leading her to rebel. Driven by the need to protect Hummy, she involuntarily transformed into the Pretty Cure warrior Cure Beat, shattering her pendant and permanently trapping her in human form as Ellen Kurokawa. Struggling with guilt over her past actions, Ellen initially rejected friendship offers from Cure Melody and Cure Rhythm. She found temporary shelter with Shirabe Otokichi, who facilitated her enrollment at Private Aria Academy alongside Hibiki and Kanade. Her personality shifted dramatically: she became cheerful, openly expressive, and developed a deep curiosity about human life, though prone to childish fears and emotional vulnerability. She gradually reconciled with Hummy and formally joined the Pretty Cure team after resolving her inner conflicts during a mission to protect a boy named Mamoru. As Cure Beat, she wields the Fairy Tone "Lary" for transformation, declaring "Strumming the soul’s beat, Cure Beat!" Her attacks include "Beat Barrier" and "Beat Sonic" using the Love Guitar Rod. Later, she gained the "Crescendo Tone," evolving into Crescendo Cure Beat with enhanced attire featuring wing-like segments and lighter hair. In *Precure All Stars New Stage: Mirai no Tomodachi*, Ellen appears as Cure Beat alongside the Suite Pretty Cure team. She participates in battles against fragments of the villain Fusion in Yokohama, collaborating with other Pretty Cure teams, including Smile Precure. During a harbor-front skirmish, she and her teammates unleashed their special attacks to defeat Fusion’s spawns, supporting the newly emerged Cure Echo in confronting the reborn Fusion.