To tell of her past, we must go way back to her ancestry.
About a century ago, a race of demon ponies, known today as The Warriors, tried to overthrow the Princess, concour Equestria, and plunge the planet into a period known as The Dark Carnival. However, they were defeated by Princess Celestia and the Elements of Harmony. They were banished to the moon in eternal exile, but not before they vowed that "The Last One" would rise and defeat Celestia and carry on The Dark Carnival.
Only one Subjugglator managed to survive, Windrider's Unicorn grandmother, Looking Glass. She moved to Canterlot and married a colt named Eternally Silent and soon gave birth to Water Lily, Windrider's Unicorn mother.
Water Lily grew up and married Windrider's father, an Pegasus scientist named Darkhoof. Windrider was born a Pegasus in 1991, and two years later, her younger Unicorn sister Moon Shadow was born.
Tragically, in 1993, Windrider's father disappeared after a lab accident(case files showed he was experimenting with an experimental chemical compound called "Formula 666".) . He was pronounced dead and a small funeral service was held. In an attempt to keep Windrider and her sister from following in their father's footsteps, Princess Celestia supressed their memories of their father. He was completely and utterly forgotten.
In 1997, when she was six, Windrider and her sister were walking home from school. About halfway there, they were attacked by what Windrider was described as "a tall, skinny pony with no face and these moving strings coming out of his back".
Moon Shadow was fatally injured, her neck broken and her spine smashed. Windrider wasn't badly injured, only suffering a broken leg. Moon Shadow was unable to be saved, as the doctors(including Doctor Whooves and Nurse Redheart.) knew that the best thing for her was to let nature take its course.
Windrider became depressed, often becoming Discorded, and stayed inside more often.
In 2005, when she was twelve years old, Windrider's depression had reached its peak. She took a knife and begin to cut her front leg so she could bleed to death, but was stopped by Water Lily. "I have no reason to live anymore! I'll never find anypony like Moon Shadow! Nopony can replace her! NOPONY!" she sobbed at the hospital.
In an attempt to console her, Water Lily turned to Zecora, Windrider's first friend other than her sister. Zecora spun many tales about brave dragons, sly griffons, and even the one Windrider saw when she was six(This was the Slender Pony.).
Windrider spent more time outside with Zecora, and soon found her special talent: Making kites. Windrider made beautiful kites of many variety, including dragon, griffon, and even manticore kites. She even crafted a kite of the Princesses.
She also learned how to fight, becoming the top student in her martial arts class. She soon rivaled even Applejack in strength. She became an excellent flyer, graduating from Cloudsdale Flight School in 2006. Her Cutie Mark was earned: A blue skull with wings.
In 2007, when she was sixteen, Windrider left her mother and moved to Ponyville, where she got a job immediately as a baker next to the Cake family and Pinkie Pie. She built herself a tree house on the edge of the Everfree forest so she could be closer to her friend, Zecora.
A year later, in 2008, she encountered the Slender Pony, the thing that had killed her sister. Enraged, she attacked him, only to be met with pleas for mercy. As it turns out he was her long-lost father, mutated after the accident at the lab. He killed Moon Shadow by accident, not knowing she was his daughter. Soon, Windrider accepted him as his father, and her depression lifted a little. Only a little.
Finally, in 2012, she turned 21. She soon became friends with ponies like herself, including Phazon, Grimshot, Blaze, and Nightmoon. She also became friends with Rainbow Dash and Fluttershy, and even the Cutie Mark Crusaders. She became very happy.
However, in the same year, she challenged Rainbow Dash to a race after Twilight confirming that her speed could rival Rainbow Dash's. Dash teased her during the race, and Windrider flew with the speed five times that of a Sonic Rainboom. She fainted after the race. While taking a blood sample, it was found that her blood was a mutated candy red, forcing Water Lily to tell Windrider's friends about her family herataige. Windrider woke up as a? Warrior and killed the Doctor, then proceeded to maim Fluttershy, Vinyl Scratch, Rarity, Twilight, and Lyra before she was brought back to reality by her friends.
The fact that she, the timid Pegasus Pon that was trusted by her friends, family, and other citizens of Ponyville had actually done something so horrid will haunt her the rest of her life.