Arya was born the seventh (and youngest) daughter of Minerva and Tristan Tritan. Yep, her dad's name was planned that way. Minerva met Tristan while the former was waiting in line for a chance to see a talent scout. Well, Tristan heard the voice before he actually saw her. Because at that moment, Minerva had realized that her life, seven years of magic and then following the crowd of people who could sing as well as she could for the slightest of slight chances for fame and fortune, was not worth it. So she sang one last song with the people she had gotten to know during the two day build up to see the guy, and left. Running into Tristan in the process.
It was love at first sight. As it turned out, both Minerva and Tristan lived in the same place, Avonmouth, just off the coast. They had moved further inland after completing their seven years at Hogwarts-Tristan in Gryffindor, and Minerva in Ravenclaw-and were attempting to find non-magical jobs. Tristan, after asking Minerva to marry him a few years later, ended up in a Ministry position, in the hopes of moving up the ranks. Minerva continued to follow her dream of singing, without following the crowds, and became a successful musician.
Even more years passed, and Tristan and Minerva now had a large family. Six girls, all of which as they grew older, showed signs of magical ability. And then there was Arya, the last. She was different than the others. Arya showed not only that she could perform magic-she accidentally made her oldest sister's hair turn green-but that she was a Metamorphmagus. The only one in her immediate family (as her mother had asked her side of the family, it turned out her great-grandmother was one as well).
At eleven, just like her other six siblings, Arya got the acceptance letter from Hogwarts. This year the trip to Diagon Alley and King's Cross would be different. She'd get to do what her siblings got to do for the past eight years (her oldest sister completed Hogwarts the year previous).
Upon arrival at Hogwarts, Arya found out that she was compared against the rest of her family. A lot. She didn't fit neatly into Ravenclaw or Gryffindor like her sisters. The hat saw her ambition, her courage daringness to get there, and placed her in Slytherin. She was the trouble maker, the one always trying new things.
There most likely would have been consequences at home for all of her mischief, but tragedy had struck. Minerva had just been out shopping for gifts for when her daughters came home for the Christmas holiday when a car hit her before fleeing the scene. Rumors had flown afterwards if the driver had a feud with Tristan at the Ministry. Minerva didn't make it.
That Christmas, there was no tree, no music, no caroling. Everyone was in mourning. Arya rebelled even more than usual, hoping for some kind of attention to lighten the mood. It didn't work.
Losing Minerva changed Tristan, who became strict. He had gotten wind of what Arya did at school, and they were no longer on the best of terms. He wouldn't even allow music at home anymore, or fashion experiments, or letting Arya practice her Metamorphmagus skills.
By the age of 16, Arya had had enough of all of the rules. She packed up and took the first train away from her family's house. Eventually, Arya ended up on the far outskirts of London, out of Muggle money. She had absolutely no idea what she had gotten herself into.
She managed to get to London eventually, got her supplies for the next school year, and made it to King's Cross without a hitch. The hardest part? Hiding from her sister, and ignoring the mail sent by owl almost every day, telling her to come home.