Every night she dreams that she is fighting demons and monsters and things she doesn't have a name for. She dreams that she is someone else. A slave. A princess. A girl in school in a sunlit city. Always fighting, usually winning, always strong.
Mel herself isn't strong. She's never been strong. She's small, slight, needs to wear glasses, and is weak even in comparison to normal humans her age. The fact that she lives in a world full of mutants and pumps just emphasises that fact. But she dreams, every night she dreams that she's fighting, and that she's strong. And she wants to be the girl she dreams of.
Harth Fray, Mel's twin brother, is strong. He's strong and fast and heals quickly, and he loves his sister. When she asked his to teach her how to fight, he aquiesed, and was surprised not by Mel's physical prowess, which is less than average, but by her instincts. Her body doens't move in the way she wants it to, but he never needed to teach her to fight, just let her use him as practice.
But nevertheless, he protects her. When the twins were fourteen, Harth had persuaded Mel to go out on a grab for him, and they were ambushed by a lurk. Mel, whose dreams often feature lurks and who is terrified of them, ran. Harth fought it off, and ran with her, picking her up and carrying her to protect her.
That night, their eldest sister and legal guardian, out looking for them, ran into the same lurk and was killed. The twins were left alone, and had to fend for themselves. Mel got a job working at a tav, Harth became a professional thief.
When they were nineteen, first a crazy bald man and then a goat legged demon turned up and informed Mel that she was the Vampire Slayer. The one girl chosen from her generation to fight demons. Which made sense, given her dreams, but didn't explain the fact that she isn't strong, or fast, or anything other than physically human, if an above average fighter due to her intense training with Harth.
Harth. Who is strong, and fast, and just generally good at stuff. It doesn't take the twins long to figure out that he has the Slayer strength, and she has the dreams.
The rest follows canon fairly closely. Harth and Mel have to wage war on the vampires, now their sister Erin, is not only a vampire but the prophesised One Who Will Lead, who summons a giant snake to bring the demons back into this world. The twins stop her, but not before Urkonn kills Mel's friend Loo to try and persuade them to fight.
Now twenty, Harth and Mel are a kickass team of a Vampire Slayer in two bodies.
Mel stands 5'3", slight and feminine, with wavey black hair and blue eyes behind glasses. She can stand up for herself in a fight, and has been learning magic since the twins found the Watchers' Library. She's about Willow season 2-3 standard.
Harth is a couple of inches taller, also with black hair and big blue eyes. He sports a long scar on his left cheek, and carries the shiny red scythe of Vampire Slayer doom. He's very protective of his sister.
There's no creepy twincest or horrid angsty doom about these two, at least compared to the Milliways versions.
Summary: Main differences from MilliFrays:
Mel: Has no superpowers but does have Slayer senses. Has black hair and glasses. Can do a little magic.
Harth: Isn't a vampire. Is twenty years old. Has Slayer strength and speed but no dreams. Doesn't wear glasses.