Eloise Sutton was born in the heart of World War II in Birmingham, England.  Growing up, all she knew was that her father had been a British soldier who'd died during the war.  She lived a comfortable if mundane life in Birmingham, raised by her mother Elizabeth.  Elizabeth remarried a carpenter involved in war reconstruction, and Tom Halliwell was a decent stepfather to the fair-haired waif who'd been fathered by a dead warrior.

All of that changed when Eloise began to become a woman.  She had periods where everything seemed to slow down, like a movie projector running too slow.  She'd reach for something and find she'd dashed across the room.  As she entered her teens, she knew only that she had become very, very fast.

Then Colonel Brighton from M-I6 contacted the Halliwell family.

Her father, it turned out, was no ordinary soldier.  He'd been the British super-soldier Swift, one of Her Majesty's stable of paranormal heroes during the War.  And like many of them, Swift - Jack Sutton - had not survived the war.

Sutton's powers had been the result of scientific and chemical alterations.  It hadn't been known how this would affect him long-term; and certainly not how it might affect his descendents. But then, M-I6 hadn't known about his relationship with Elizabeth... until it was too late.  Once they found out, Sutton was deployed on the front lines... and reported killed in action during a secret raid into Nazi Berlin.

Since Eloise had been born out of wedlock, and her parentage wasn't completely certain, M-I6 had kept tabs on the girl as she grew, but otherwise left the Halliwell family alone.

Until Eloise began to get fast.

Now, they met with the family and gently revealed the truth of the matter.  They called upon Eloise to serve Her Majesty as her father had - and be trained to use her powers for good.  And for her own safety.

The young girl, always curious about her real father, and raised with a strong set of ethics by her mother and stepfather, agreed.

For the next six years, her education was supplemented by M-I6 training.  Rather than focus on the combat aspects of her speed - although they did train her in combat - they trained her how to use her speed to evade detection, deliver illicit packages, and scout areas that no normal operative could.

During all this, of course, they also strove to let her lead as normal a development as possible.  This wasn't always successful.

Finally, she was given the callsign "Vosper" - named after the ultra-fast torpedo boats of the war - and given her first assignment at the height of the Cold War.

Vosper was going to America.