The first time he beats her, it is horrific.
She is not splattered with blood. She is soaked with it, so much that it looks almost comical, as though a bucket of red paint has been dumped on her head. Her arm is broken, and she is fairly certain that her nose is too.
She makes the mistake of crying out, which only seems to fuel his rage. He smashes her to the floor, and his repeated blows fall harder every time she screams. Finally, she is silent, taking the pain noiselessly, tears shimmering in her green eyes.
When he leaves her, quivering on the floor, she slowly drags herself to her feet (she's amazed that she's able to stand - she must be more resilient than she thought) and limps off of the Division grounds. She intends to head for the Fourth Division, but only ends up making it as far as the Tenth before she collapses, her breathing ragged.
This is how Ise Nanao and Matsumoto Rangiku find her half an hour later. They are walking, and she can hear them laughing and joking.
"Oh, come on, Nanao-chan! Admit it, you've thought about it too!"
"Yes, but at least I've had the good sense not to say it out loud."
"But-"
"Wait. What's that?"
"It looks like- oh kami, I think it's Nemu."
A pause. "It is. Is that... blood? Oh, kami, she's hurt, Ran! Go get someone from the Fourth!"
"Okay! I'll go find Isane!"
There is the whiff of ozone that always accompanies shunpou, and Rangiku is gone. For a few minutes, there is only the blue glow of Nanao's kidou. A budding kidou expert she may be, but she only has a rudimentary grasp of healing spells; just enough knowledge to stop the worst of the bleeding and get Nemu more or less stable.
Another flash of ozone, and Rangiku is back with the silver-haired Isane and two unseated members of the Fourth in tow. Isane is all business. She calmly runs a diagnostic spell and orders her subordinates to load Nemu onto a stretcher.
Isane sends a message ahead of them, and when they arrive, a veritable militia of healers has gathered, armed with bandages and antiseptic. They set her arm and get her a fresh shihakushou - hers is too battered to salvage - and someone cleans the blood from her face and hair. Soon, all that remains of her beating is a white cast and sling encasing her right arm, and a large bruise covering her entire cheek.
Who did this? they ask. Who hurt you? She lies easily, deflecting their questions with quiet shakes of her head and soft words. It really was her fault, after all - she had accidentally dropped a rack of test tubes that her father needed for an experiment. She was to blame. She had provoked him, set off his temper.
All her fault, always.
Nanao and Rangiku help her back to her division, Rangiku chattering a mile a minute and Nanao mostly silent, concern written plainly on her features. They hand her over to her father. His eyes glint dangerously, his voice acidic to Nemu's ears as he thanks the girls for locating his wayward daughter and providing her with medical care. They smile and nod, but she sees it in their eyes: they know that he is responsible for her injuries.
And even scarier, Mayuri-sama knows that they know.
They leave, and she is afraid that he will hurt her again, but he only sighs and turns back to his experiment. "Go make yourself useful, will you? Show the recruits around, and then go help the Seventh seat take inventory in Storage Room B."
"Yes, Mayuri-sama." She bows and leaves the room.
As the years pass, she develops the ability to endure the harsh beatings without screaming, and then without crying, until she eventually just stares with dull eyes, calmly waiting for it to end. It is not unusual for Nanao and Rangiku to find Nemu lying on the ground in a heap somewhere between her own division and the Fourth, soaked in blood and too exhausted to move another inch. They bring her to Isane, who shakes her head sadly as she heals cuts, tapes fractured ribs, and splints broken bones. Afterwards, they return her to her father. It makes them uneasy to do so, but Nemu assures them that all her injuries are her own fault and that she will be perfectly safe.
Yes, safe. What a lovely word.
He owns her, and he reminds her of it often. I created you, and I can destroy you whenever I wish. You'd do well to remember that.