Rosetta Passel, born in Belgium and raised in France, gained recognition as a prodigy in diabolo manipulation from a young age. By age nine, she won her first diabolo tournament and earned the moniker "The Diabolo Machine" for technically flawless yet emotionally distant performances. This perceived lack of passion and audience connection initially led to her dismissal from Kaleido Stage.

Her development ignited when Sora Naegino helped her rekindle the joy of performance, teaching Rosetta to prioritize audience connection over technical perfection. This collaboration forged a strong sisterly bond, with Rosetta coming to view Sora as a role model. Evolving from an isolated perfectionist into a more expressive and passionate performer, she permanently rejoined Kaleido Stage in the second season. Demonstrating dedication to growth, Rosetta ambitiously trained in trapeze to expand her skills beyond diabolo.

In the OVA *Kaleido Star: Good da yo! Goood!!*, Rosetta features prominently in "The Amazing Attempt at the Diabolo," instructing Ken Robbins. Her teaching style blends candid, comedic bluntness with genuine expertise, covering fundamentals like string adjustments, basic spins, and advanced tricks such as "The Elevator," while emphasizing performer trust. Her pragmatic nature surfaces as she invoices Ken for the lessons, citing Kalos’s refusal to cover the cost.

Rosetta’s relationships extend beyond Sora. OVA end-credit scenes show her voicing aspirations to mature and perform alongside Sora, joined by Marion and Jonathan envisioning their futures. She also appears in a haunted house scenario with Sora, Anna, and Mia, reacting fearlessly to Ken’s scare attempts. Signifying her profound commitment to performing arts, Rosetta gains the ability to see Fool—the stage spirit visible only to those deeply dedicated to Kaleido Stage—by the end of the second season.

A recurring detail reinforces her familial background: Rosetta mentions her mother across the series.