"The true name of Cinderella's heroine. "Cinder Ella" was a derisive nickname, born from her hard work always leaving her dusty and sooty. After marrying her prince, she became "Princess Ella.""
—Persona 5 Royal compendium
Cinderella is a fairy tale adapted from a Greek folk tale. The most famous version, by French writer Charles Perrault, introduced the iconic "glass" slippers, as well as the pumpkin carriage and the fairy-godmother.
The tale of Cinderella follows a kind, beautiful young woman who is forced to be a maid for her wicked stepmother and two equally vain stepsisters. A ball is held by the kingdom's prince to find a bride, and her stepfamily sabotages her chances to attend the ball. Her fairy godmother then appears and grants her a beautiful dress, allowing her to go, but warns that her spell only lasts until midnight. At the ball, she attracts the prince, but is forced to leave just before midnight as the spell begins to wear off.
She leaves behind one of her glass slippers in her haste, and quietly returns home before her stepfamily can find out she left. Having found the slipper, the prince travels the kingdom to find her. He has every maiden he visits try on the slipper, but none are a perfect fit. Arriving at Cinderella's house, the slipper fails to fit on the stepsisters, but it fits perfectly on Cinderella. After producing the other slipper as proof, Cinderella marries the prince and lives happily ever after.
Cinderella is not the actual name of the character (which, in some adaptions, is Ella), but a nickname from her stepfamily as she was often covered in cinders when sleeping next to a fireplace to keep warm.
Ella is the third-tier Persona of Sumire Yoshizawa. Its Trait is Veil of Sunrise, which greatly increases the chance of an ally not being Downed when attacked.
In order to unlock it, max the Faith Arcana Confidant with Sumire and talk to her on the 2nd of February at Kichijoji for a hangout in her room. Vanadis will then transmogrify into Ella.
Due to the date of Sumire's third awakening being the penultimate day of the game, it is impossible to use it prior to February 3rd, where combat is last available. This applies even for New Game Plus.