He calls her 'Tsubaki' when they first pair up. Introducing himself to others, he'll say, "I am the Great Black*Star- the man who shall surpass God- and this is my weapon, Tsubaki." And that is her cue to shift from his hand and land, crouched, head bowed, at his side.
After some days of this, he confronts her. "Tsubaki's the best weapon," he says, "so why do you bow?"
"Because, I am yours," she slowly replies, confused. She is a tool for him to become greater than God. She is his. "When you say 'my weapon, Tsubaki', I feel it is inappropriate, otherwise."
"But Tsubaki's the best weapon," he repeats, like she is a slow child, and maybe she is. "If the best weapon's taller than God, then all weapons'll wish they were tall."
She only smiles and shakes her head slowly.
They reach a compromise: he, saying "-and this is Tsubaki, my weapon," and she, standing with her head high.
Their friends address her as 'Tsubaki', because that is how he introduces her, because that is the given name her family had bestowed when she was born. She does not have a nickname. She doesn't need one. She doesn't understand the need for one. Her name means 'camellia', and she is happy to be the scentless flower, and does not want another name to mask it.
When they are alone, in the privacy of their shared home, her name is 'Tsubaki'. It is how her family addressed her across the sea, and he is respectful by calling her the same, no matter how familiar they become with each other.
They hadn't dated. Or even flirted, that she's aware of. He just says to her, after a long mission and flopping onto the tatami, "Tsubaki?"
"Yes, Black*Star?" she answers, staring peacefully at the rafters in the ceiling.
"You should marry me."
She blinks, looks over at her meister, and sees him looking at her, very serious, very mortal. He's younger. He will surpass God. He helped find her brother. His heart is in the right place. But she wants to ask, 'Why? Why me? Why so suddenly?' because she is not the best weapon- she is a shadow weapon, a facsimile of real weapons, weaker than real demon shurikens and real demon katanas and real demon chain-scythes, a Jack of all trades and master of none- but she simply says "Yes, Black*Star," and watches his eyes light and smile ignite as he glances to the rafters.
He wears a a look that says, 'With this, I have surely surpassed you', while gazing at a point beyond the ceiling.
It's a private ceremony in her father's house. After, they are expected to eventually consummate the marriage, but although she's been proven to have an abundance of sexual hormones, she is extremely nervous. They haven't done anything but hug before.
Even as he leans in to her, he calls her by name. "Tsubaki, I'm going to kiss you."
Her blood sizzles and crackles when their lips meet.
"Goodnight, Tsubaki," he says, and though he's been known to peek at her nude body in the bath, he settles in beside her for a quiet night of sleep. And she knows, finally, that she will love this man until she dies, and will be hard-pressed to stop even in the afterlife.
When they are intimate, when he drives into her, when she writhes over him, when his seed flows into her and mixes between them, he calls her 'Tsubaki'. But once, when her hair is gently caught up between his chapped fingers as she softly attempts to take him into her mouth, he calls her 'Sue'.
It surprises her- startles her, truthfully, making her pause. But he repeats it, quietly. "Sue, don't stop."
She doesn't ask. It's strange, but doesn't bother her as much as she thought it would. He continues to call her by her given name like it had never happened.
It slips out again, randomly, as he's kissing her. Again, when he is entering her from behind, mouth hovering close to her ear. Always softly, like a secret. "Sue."
Finally, when she asks, "Black*Star, why do you sometimes call me 'Sue'?" she's surprised to see him look somewhat embarrassed.
He explains it like this:
"Tsubaki's a spotlight, and everyone will say 'Tsubaki' when they want her attention. Because to say her name is to have her light shine on him.
"Tsubaki is Sue... when all I can do is give the stage to you."
As she digests all of this, recalling the moments he's called her by his private nickname and flushing with tangled emotions, he adds in a conspiratorial whisper, "You make me forget my lines, sometimes,"
He calls her 'Sue', when she has his attention, which is probably the highest compliment, from him.