Sometimes they are Saiph, and sometimes they are Sirius.
She was born Saiph, all dark hair and soft skin and was as bright and as sharp as sword from her first days. She was brighter than her brother Regulus, quicker than her cousins.
When she was 8 she realised she wasn't always Saiph. She explained it to Reggie who didn't know quite what she meant but suggested that Saiph find a name on the family tree to use when she wasn't Saiph, and so not-Saiph searched and searched and made a list.
"Saiph," Reggie said. "These are all boys' names."
"I feel like a boy though," replied not-Saiph and so Reggie nodded and pointed to one name on the list.
"Sirius is a good name. Sirius is the brightest star of them all," said Reggie and Sirius smiled at his brother.
At dinner that day Sirius interrupted Walburga when she called him Saiph and when Walburga looked confused Sirius let Reggie explain it to their mother. When Reggie was done Walburga smiled and said "We'll have to get you some new robes then."
When Saiph got her letter she was elated to see that for her name it did put "Saiph/Sirius Black".
She went to Diagon Alley and got clothes for Saiph and for Sirius and got all of her things and an owl she named Salmacis. No one laughed. No one asked why she needed two sets of robes and Madam Malkin slipped her an owl-order address "for when you hit puberty dear".
On the train Sirius met James Potter who seemed to smile at every pretty or handsome or attractive wix who walked by, Remus Lupin who seemed somewhat scared and Peter Pettigrew who seemed unsure and confused. They became friends. They also met Lily Evans whose eyes sparked like hexes and Snivellus who kept his tatty robes wrapped tight around himself as he marched out of the compartment with Lily.
Once Sirius was sorted and seated with his new friends they enjoyed the feast and eventually headed up to the dorms. The prefect had to give Sirius a nudge up the stairs and when Sirius was safely ensconced in his four-poster bed he relaxed for the first time since he'd passed through the barrier to Platform 9 ¾.
The morning however was different. Saiph woke up, Saiph got dressed and Saiph headed downstairs, her hair up in a ponytail. She walked all the way to the Great Hall before James noticed and asked "You ok mate?"
Saiph nodded and said "I'm Saiph today. Are you still my friend?"
James laughed loud and clear, gave her a pat on the back and said "of course" and Saiph let out a breath she hadn't realised she'd been holding.
When Remus and Peter came down James explained it to them with a glance to Saiph to check it was alright and when Saiph nodded and he had explained Remus reached out to pat her arm. "It's ok;" he said softly, "I don't feel right in my own skin either sometimes."
Saiph smiled shyly and was glad to have such a group of friends.
The day passed uneventfully until they were waiting outside Transfiguration and a Ravenclaw boy, name now forgotten, pointed and laughed at Saiph.
James started forwards but stopped when Saiph gripped his arm in a tight grip. "This is my problem," she said and drew her wand.
Contrary to later stories it only took two hexes to fell the Ravenclaw not six or seven or twelve. But the Ravenclaw was pinned to the floor, eyes full of fear when Saiph, still only eleven, looked down at him and said, "Today I am Saiph. My name means sword."
The Third Year Hufflepuffs waiting for Charms cheered. The Sixth Year Slytherins smiled. And the Ravenclaws left the boy on the floor, pulling books from bags to give to Saiph as a peace offering. Saiph only took one.
Gender in the Wixen Community and How it Affects your Day-to-Day Life
No one challenged them again.