Ellie's father died and her mother remarried, which eventually led to Ellie living on the streets.
In 2009, Ellie was living on the streets of London. She befriended other homeless people and learned of the "Night Dragon" which took homeless people away. She had seen a sign advertising a jazz show starring one "Ellie Faber" and decided to adopt that name.
In 2011, Ellie met Clyde when he gave her some money for food. The next day she saw him thrown out of the museum as a result of the curse. That same night she found Clyde in the heavy rain. She took him to a homeless camp in a tunnel where they stayed the night. Clyde introduced himself as "Enrico Box" as his real name was cursed. As Ellie was used to using an alias, she immediately realised that Clyde had taken the name from a pizza box.
Later, Ellie took Clyde to a homeless shelter where they encountered a woman who (correctly) indicated that Clyde was cursed. This made his persona not welcome at the homeless camp, so Ellie and Clyde found another place to shelter from the nightly rainstorms.
They bonded and Clyde drew a portrait of Ellie, saying he could do art to get the two off the streets. A grateful Ellie kissed Clyde and went to get a coffee for them to share. She did not see Clyde again as his friends had broken the curse and had to take Clyde back to Bannerman Road to defeat the alien that had cursed him.
Once the curse was lifted, Clyde searched all over London for Ellie but discovered that it was an alias and that she had been taken by the Night Dragon, which was actually a haulage company which travelled around Europe and would occasionally take the homeless to new cities. Ellie's acquaintance, Max, explained this to Clyde and said it was a "second chance."
Ellie was a sweet girl. She had been hardened by the streets but that didn't stop her from opening up to Clyde and making jokes. She grew very close to Clyde in the brief time they were together, developing romantic feelings for him. (TV: The Curse of Clyde Langer)
Behind the scenes
At the conclusion of the story, Clyde indicates to Sarah Jane that he'll never stop searching for Ellie. This is one of several plot threads introduced in series 5 which were left unresolved due to the series ending.