Kubja was a servant girl of Kans with the name Anekavakra/Trivakra. Her face was very beautiful but her body was deformed with a noticeable hunchback. In Brahmavaivarta purana she was an old , deformed lady walking with a stick.
When Shri Krishna and Balarama entered Mathura, they met Kubja and Krishna sweetly asked about her and she told them her name and told that she used to apply fragrant pastes on Kans’ body and he liked her pastes only. She offered fragrant paste to Krishna and Balarama. Krishna touched her with his two fingers on her child and made her straight and she became very beautiful and her body became straight. She was overwhelmed and invited them to her home to host them but Krishna declined the offer politely and after sweetly talking to her and telling her that they would eventually come, she left and Krishna and Balarama laughed.
In Brahmavaivarta purana, Kubja was Surpanakha reborn as she in previous life did Tapasya to get Shri Ram as her husband. Brahma denied Surpanakha her wish but he told her in next life she would become his wife and then as Kubja, she became one of the wives of Shri Krishna but this story is not in Bhagvat Purana.