Fili-Second bio

Name: Jackdaw "Jiffy" Zoccolo
Alias: Fili-Second
Power: She can control her heart-rate, and when it goes a certain speed, she can transform into a super speedster, and run at supersonic (and possibly faster) speeds. Also has faster healing.
Weakness: The cold; her body gets slower, and if her accelerated heart-rate goes below a certain speed, she’ll turn back into her civilian form; where she has a cast on her leg and is unable to go faster than a limp. Tends to act before she thinks and takes risks; believing she's invincible because she can dodge a bullet, getting her into trouble when traps are set. 

About: 
Apart of one the Maretropolis crime families, but has a love for running; born from when she saw her first horse race. She enters a race, making an opposing mob lose a lot of money, so they get revenge by breaking her leg. Neither her family, nor the Power Ponies know that she works with both of them.

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During a freak storm, 5 ponies are struck by lighting and gain superpowers.
Jiffy was running in a horse race at the time, and just as the camera took her picture on the finish line, she was struck. The lightning connected with Jiffy and the camera, and reversed her colours. It left her in the unique position compared to the rest of the Power Ponies, that she is able to transform into her alter-ego. None of the Power Ponies know that she is the daughter of a major crime boss, and her family doesn't know that she is Fili-Second.