Clara Oswald was dubbed "the Impossible Girl" by the Eleventh Doctor because he repeatedly encountered her across different timelines and identities, despite having witnessed her die on multiple occasions. Her consistent reappearance after death defied his understanding and experience, making her seem truly impossible.
The Doctor coined this term after realizing the profound anomaly of her existence. He had encountered her in distinct forms, each time believing her to be a unique individual, only for her to tragically perish. However, she would then appear again, in a new context, but undeniably the same person.
Specifically, the Doctor's investigation began after two key instances where he saw her die:
- Oswin Oswald: His first encounter was with Oswin Oswald, a genius who had been converted into a Dalek in "Asylum of the Daleks," sacrificing herself to save him.
- Clara Oswin Oswald: He then met Clara Oswin Oswald, a governess in Victorian London in "The Snowmen," who also died while helping him.
The realization that he had run into the same woman when she had died twice sparked his intense curiosity and concern, leading him to investigate her identities and label her "the Impossible Girl." This mystery became a central arc during his eleventh incarnation, as he sought to understand how one person could defy fate and continually reappear in his life after multiple seemingly final demises. It was this seemingly impossible recurrence that made her a unique and perplexing companion.
