They have been asked many times about their relationship, and most people just assume Bella was forced by her parents to marry Rodolphus.
The truth is far more romantic that Bella likes to admit to anyone, but she always keeps the memories close to her heart.
When she meets Rodolphus he's a boy of eight and she is seven, and both are bored out of their mind by the dull reunion of their parents and other associates. The children around Bella don't get her, not even Andy who is her most beloved sister, and she wanders away. Rodolphus chases her, but not out of his own wishes.
"Father told me you are wild," he tells Bella with all the seriousness a boy of his age can muster. "He says not to let you out of my sight."
She does notice the way he looks at her hair, as wild as he's been told Bella herself is. She giggles because Rodolphus might be slightly annoying but he still is fine to look at, not like the dumb Lucius who looks like Narcissa's dolls. Rodolphus is dark in all senses of the word (as far as little Bella knows), with his black hair matching his brown eyes and the air of strangeness around him falling like heavy rain on an evening.
"Then don't," she says and runs and he goes behind her matching her step by step.
It goes on like that for years, with Rodolphus trying to be Bellatrix's keeper and her never letting him because she's a witch and for them the power must be more important than gender, even if her parents think otherwise.
The first time they kiss is at Hogwarts. Bella is trying to get to the kitchens without being followed by Andy, who thinks they should always be together because they're sisters and because they're Black. Bella only wants to eat some cake without thinking about murdering Andy and leaving her body on the lake to feed the Giant Squid.
Rodolphus saves her by telling Rabastan to stall Andromeda with questions about their Potions essay. As if Slughorn cares about whether or not Rabastan writes with silver or gold ink.
"Poor Andromeda," Rodolphus says, looking ahead and walking to the kitchens with his hands in his pockets. "She just wants to be with her big sister."
Bellatrix huffs because Rodolphus doesn't know how hard it is to escape from Andromeda's incessant chatter.
"I don't want to be a big sister," she confides in him as they pass the portrait of a particularly fat monk. "I want to be me."
"You're already you," Rodolphus tells her with a casual air. For hi,m it's not as important, but to Bella her identity is everything.
"You don't understand, Rod," she exclaims, slashing in front of her with her wand and sending a vicious spell onwards to collide with a pillar. Good thing Hogwarts endures. "I want to be in a place where people see me and they don't think about my money or my surname."
It's the most romantic thing when he stops her and makes him look into his eyes.
"You are you," he says, and as she stares into the brown eyes that have been chasing her for almost a decade, Bellatrix knows what he means.
She smiles at the memory one last time before the dementors take it away.