Their dynamic is an odd one. Namely, if you replace 'odd' with 'vitriolic'.
"Bring those files over here. Do it now, you useless hollow!" Kurotsuchi barks from one side of the lab. From the other, Szayel Aporro Grantz tosses a stack of paper held in a folder bound by rubber bands; they slide across a stainless steel counter and come to a measured stop beside Kurotsuchi's left hand. He grunts slightly, before pulling open the files and reading the front page. Grantz returns to his own work in typing up mindless page after mindless page of equally mindless scientific information that only he, Kurotsuchi, and Kurotsuchi's daughter Nemu would understand.
Szayel knew he should still be in the sands outside of Las Noches, waiting for the moment in which the Captain's blade would finally, finally kill him. It felt like an eternity, and definitely drove him mad for so long a period of time. Instead, the blade (when it finally reached his body) didn't pierce his heart. No, only the area beside it, and his hand of course. And then he spent another eternity in his mind watching Kurotsuchi return and administer an antidote, and time slowly grew human again. He couldn't control his body for some time afterwards, while Kurotsuchi dragged him away and back to Soul Society. He recovered, locked away in the bottom of the Twelfth Division's labs, until his sense of time returned to normal and he could control his movement again. And then he was instructed, by who he would come to know as Nemu Kurotsuchi, the woman that he almost killed and who he had 'rebirthed' himself from (and who, indeed, had almost killed him as well), that he was a test subject of Mayuri Kurotsuchi, a name he would come to loathe with a very cool, controllable vitriolic fury.
He is a slave for Mayuri Kurotsuchi (and the Soul Society itself) now. This, understandably, fills him with a disgust so total as to be sickening. He abhors the shinigami still. And it's no wonder why.
"Kurotsuchi, there is a discrepancy between your formulas and mine. One of us is wrong." Szayel's voice comes cold and clinical as he sits in front of the huge computer. He can control his pride enough to stay alive and do his work, if only to formulate a plan for escape that will come to fruition years upon years from now, but he will not call Mayuri by his title and indicate that the shinigami is superior to him. This has, of course, been a major problem between the two scientists.
"You refer to me as Captain Kurotsuchi or Dr. Kurotsuchi, Grantz," The shinigami snaps in his ear as he approaches, and when Szayel does not make eye contact or acknowledge his presence, grabs the hollow by his pink-tone hair and throws him to the floor of the lab, before sitting in the chair himself. Szayel, ever-tempted to snap off another witty comment, refrains for now as he stands and tugs, uncomfortably, at the reiatsu-sealing collar bolted into his neck and spine. A twinge; his left hand goes numb as a nerve is pinched, but the feeling begins to return as he stands, statuesque, at Kurotsuchi's left.
Mayuri is silent for a long period of time while reading through the formulas, comparing them to his and to Grantz's. Szayel straightens his white robes as he waits, because there is nothing else to do but wait for Kurotsuchi to finish his examinations.
"Yours are wrong," Kurotsuchi states in a deadpan tone, shoving a heavy file off of a desk and down at Szayel's feet. "Do them again, and get them right this time, moron."
Szayel dutifully picks up the file, heavy as it is, and waits until Kurotsuchi turns his back to alter a few hastily-scribbled words on a nearby sheet detailing incredibly complex formulas for an experiment. Sabotage is not in his nature, but Mayuri Kurotsuchi deserves it.
Their days are spent together in the lab, where Szayel lives, breathes, and sleeps menial labor. He is an experiment, but in what, he doesn't know. He hasn't seen the sky in what feels like ages. How long has it been since the war? About three years, they tell him. His only comrade is the slowly slipping sanity and the voice of Fornicaras, and there is a very terse alliance with Nemu Kurotsuchi that extends no further than her delivering his meals (which aren't that big, or great, but which are enough to live off of) at night when he sleeps in his small, wet, uncomfortable cell. His relationship with Mayuri is a very slight scientific respect for one another, but pure and total hate on any other level.
Their nights are spent working, in pure and total silence. Their understanding of one another over these three years has become so great that they rarely need to speak. Kurotsuchi can lift a hand and Szayel will see what he's working on and know what he wants. What starts out as a hollow/shinigami relationship has turned into scientist/assistant dynamic, and from there into a scientist/scientist understanding.
That doesn't mean that they're actually genial to one another.
"I'm going to say it one more time, Kurotsuchi: this mixture is going to be explosive," Szayel instructs in his clinical-but-pompous "I am better than you so listen to me" tone of voice, standing behind and to the left of Kurotsuchi while he prepares two chemicals.
"Shut up, before I slice your tongue out, Grantz," Mayuri snaps back, his entire attention diverted to the experiment before him. His frustration with the general unhelpfulness Szayel is showing is great, and his ability to tolerate annoyances is shorter than usual today, which generally means it's nonexistent.
"You know I'm right," Szayel murmurs, walking to the other side of the lab to avoid the imminent explosion. After a minute or so, Mayuri stands and faces Szayel, a triumphant look in his eye as he holds the finished chemical, which happens to be a deadly neurotoxin with a purple tint.
"Now will you shut your mouth, Espada?" Kurotsuchi asks, as the hollow scientist crosses his arms and glares from behind his glasses lenses. "My experiment was a success, and with no explosions. Far superior to anything you've created so far."
"That's only due to your inability to let me use any of the facilities here." Grantz states this in a monotone so sharp as to almost cut Kurotsuchi off. "Are you afraid that I'll actually create something better than what you've managed so far? Admit it, shinigami; you know my capabilities are far greater than yours."
The next thing Szayel knows, a syringe full of a purple-tinted neurotoxin has been stuck into his arm, and before he can jerk the needle out, the chemicals are already taking instantaneous effect. There's the beginnings of extreme pain, while Kurotsuchi begins to jot down notes from a corner of the room.
"What is the level of pain you're experiencing, from slight discomfort to total agony?" He asks, as Szayel collapses.
There is no romance in the dynamic between the two; they don't love, they don't care for one another, and honestly, Szayel would greatly enjoy having the pleasure of dissecting Mayuri on his own lab table. But though there is no positive feelings, much less romantic connotations, between them, there is something else that isn't easy to name.
Mayuri sits in his chair in front of the huge computer screen, obviously frustrated with something. Szayel, after around three hours of ignoring his frequent growls of frustration or muttered curses at random people that he doesn't like, which is a lot of people, eventually walks over to stand behind the shinigami doctor and asks, "What's the matter, Kurotsuchi?"
"I told you to call me either Captain or Dr. Kurotsuchi, idiotic monster," Kurotsuchi snaps, before continuing with, "A recent test experiment isn't responding to a set of chemicals being administered, and I'm trying to discern as to why." Szayel sees a picture of a young man, lively brown and short orange hair, a distinctly severe expression on his face, and perks one pink eyebrow.
"Really? That's highly irregular; has this been your favorite subject? There might be a resistance built up to your toxins, if they all have that common chemical base that you work off of so regularly." Grantz notes, before hearing Kurotsuchi make a sound of annoyance deep in his throat, as he stands and marches off towards a door.
"Did you think I didn't think of that?" Kurotsuchi snaps, as Szayel walks behind him at a cool, steady pace. "I've altered chemical types, and still nothing." They walk in silence until reaching a dissection table, where the young man in the picture is strapped down, drugged with what Szayel knows isn't pain medication. Kurotsuchi never bothers with any sort of anesthetic. Everything is sterile and stainless steel, except for the man on the table, wheezing in raspy breaths. He's gaunt, pale as the coat of the Captain who walks up beside him and examines his physical nature with a scientist's eye. His eyes open, but when he looks up at Kurotsuchi's examining stare, he can do no more than close his eyes again. Probably a paralytic in effect, Szayel hypothesizes.
"What physical modifications have you put into effect so far?" The former Espada queries, golden eyes lighting up in a distinct memory of this person on the table. "Ah...wait a moment. Is this that shinigami boy I saw back in Hueco Mundo?"
"Yes," Mayuri states tersely, adjusting all the tubes and wires in the subject's body, "A Vizard, Ichigo Kurosaki, if I remember correctly. His name is of no importance though; he is referred to in this lab as subject number nine-hundred-ninety-two." He takes a new wire, a rather thick needle, and hunts down Ichigo's arm for a new, non-collapsed vein. He finds one, swelled and almost black-colored, and without real concern for the wheeze of pain that the former substitute shinigami lets out, slides it in. "An untoward comparison between myself and the former Captain of Twelfth was a very bad choice, boy. He knows that now." Szayel watches the tube fill with blood that lazily slides upwards, eventually turning a shade of pink when it mixes with the saline in the tube.
'The same shade of pink,' Szayel notes with disinterest, 'as my hair.' He then looks at Kurotsuchi again. "Your experiments on him so far?"
"The same technology in his blood that I used to monitor the Quincy's fight with you, blood thinners, experimental toxins numbers twenty-three, sixty-five, twenty-two, and thirteen, which you're familiar with, and right now, a recent injection of pain modifiers."
Szayel glances over slyly at this, watching Kurotsuchi from the corner of his eye. "'Pain modifiers'? You mean pain magnifiers, correct?"
"Of course I mean pain magnifiers. What else would I inject him with? Anesthetic?" Kurotsuchi answers, sarcastically despite himself. He hands Szayel a scalpel, and the hollow experimentally slices up Kurosaki's arm, from wrist to elbow. Not deeply, but enough to break the skin. Ichigo tenses and arches his back like he's been electrocuted, letting out a hoarse, dry scream. Szayel smirks, mumbling a "Hm," in an interested tone as he sets the bloodied scalpel back down on the tray of various horrific surgical instruments.
"And where did you get this one? I thought he was with the group of Ryoka you spoke of before? The ones you weren't allowed to experiment upon any longer?" Szayel asks, curiously, as Kurotsuchi picks up Ichigo's file and runs his long black nail along the lines of words, reading at a pace the hollow recognizes as incredibly fast.
"There was never a direct statement as to being restricted in my test subjects. The war is over, and Seireitei no longer cares for their expendable heroes." Kurotsuchi states it in a slightly annoyed monotone, but not particularly angry. Szayel walks up beside him, reading over his shoulder, and then points down at a particular line of scientific writing indecipherable to near anyone except the scientists themselves.
"Here, it says you used toxin twenty-two. Wouldn't twenty be a better choice? Fluorine would react better than mercury."
Kurotsuchi stares a moment, before handing the file to Szayel and walking over to Ishida on the lab table. He alters the chemicals and watches, waiting for the instantaneous effect that should be happening. A beat passes, and then Ichigo groans in pain, but cannot move; his eyes go glassy, and he stares blankly at the ceiling. He's not dead; the vital monitors prove he's still alive, and Szayel looks over to Kurotsuchi, walking up beside him.
"And did your effect work?" He asks, and Kurotsuchi gives a slight nod of his head.
"Yes, it appears to be so. His stomach lining should be separating from the stomach itself right now; when the paralytic effects wear off, he'll most likely be howling in pain and vomiting, which means Nemu will have to turn him on his side so that he doesn't choke on it. Nemu!" He snaps, and soon enough, the Lieutenant herself walks in and stands at attention.
"Yes Mayuri?" She asks, expression cool and detached, as Kurotsuchi gestures to Ichigo as he walks back towards his personal lab.
"Turn it on its side, so that it doesn't suffocate on its stomach lining later. Do it quickly!" He barks, as Szayel follows, almost hovering after him in a phantom-like manner. Nemu begins to turn Ichigo, careful of all the wires, as Szayel shuts the door to the lab. As Kurotsuchi sits down at his computer again, he gives a cursory glance to Grantz when the hollow passes.
"You've finally made an intelligent decision for once. Useful. Try to be less of a mindless beast and do it more often, Grantz." He states, already beginning to work again, and Szayel smirks. It's a backhanded compliment, as backhanded as they come, but it's still a grudging compliment.
"Of course," Szayel starts, tone airy, "...Dr. Kurotsuchi."
Mayuri grunts in response, though there is a distinct tone of acceptance in it, and Szayel begins to work again with a slight smirk on his face.
The dynamic between them is backhanded, vicious, insulting. But it's something else as well; it's an acknowledgment of one another's skill in their respective field, of each other's intelligence. They don't like one another, but they make a devastatingly wondrous scientific team.
Mayuri Kurotsuchi and Szayel Aporro Grantz, despite their vitriolic relationship, are the closest thing to friends that either of them have.