Demelza's family tree is completely flawless; every one of her relatives' blood in pure and clean, and the Blackwoods are determined to keep it that way. Dahlia Mason was only twelve when she was betrothed to Frederick Blackwood, the youngest of seven siblings, and she married him only five years later. At first, they loathed each other, as Frederick's brash, harsh nature conflicted entirely with Dahlia's sparkly-eyed innocence, but eventually, they grew closer over the course of ten years. Demelza was born first, a beautiful blonde-haired, blue-eyed, powerful looking child.
Growing up in the heart of London, Demelza was educated in swordplay, archery, literature and other life skills, and showed signs of precocious intelligence from a young age. By the time she was sixteen, she was fluent in six languages, a skilled archer and an avid reader; her younger sister was never as clever as Demelza, and never will be, which Demelza enjoys, a lot. She'd shown her first signs of magic long before that, though - accidentally engulfing a room in flames at six years old was quite the telltale. She started Hogwarts at eleven, and the Sorting Hat had barely brushed her hair when it yelled out that she was to be sorted into Slytherin. She left the stand with a smirk on her lips.
Demelza created a storm during her seven years in Hogwarts. She bullied, blackmailed, manipulated, lied, but most importantly, she worked her way to the top. Her OWLs and NEWT grades were all Os - in her mind, an E was completely and utterly unacceptable. She never fell in love - love is absent in her entire family. It's obviously unnecessary - but others fell in love with her (though she believed no one ever fell for her. They became infatuated. There's a difference). She used it to her advantage, of course. She promised her pure-blooded hand to several people in return for potions, test answers (
hey, she isn't a total genius), and other things, but no marriages ever occurred and she laughed as they begged for her hand.
After graduation, Demelza immediately went back to Hogwarts and became the over-qualified Potions professor, and was eventually promoted to Head of Slytherin house, which is absolutely perfect for her.