2. Mayuri doesn't talk with his victims often. They're there for a reason, he thinks. If he wanted their life story, he'd have given them a cup of tea and a chair. Since neither of those were around, he expected the victims to stay quiet or at least give a few convincing screams (with him at the head of things, this never seemed to be a problem). If they get too annoying, he severs their vocal chords and that's that.
3. The first modification Mayuri made to his body was changing the tissue in order to allow it to mutate. It was purely experimental, but her was pleased with the results. Since then, it has grown to be an obsession with the scientist as he reconstructs his body time and time again to surpass its limits. At first the pain was almost unbearable, but now he finds it strangely pleasant.
4. Mayuri resents the fact that many shinigami attest that he's a pervert for the attire he has given his daughter. He designed her primarily as a fighter, concentrating on agility and focused on an acrobatic form of hand-to-hand combat so that he would not have to deal with the mundane nature of combat. The shihakushou Mayuri designed allowed his 'daughter' to get a more full use of her legs than she might in the regular uniform, making her less of a liability in combat.
5. Mayuri belonged to one of the minor noble houses of Seireitei, the precious youngest of three. Unlike his brothers, he wasn't expected to do much and was indulged in whatever whims he had. Mayuri led an easy childhood, manipulating his parents to get him what he wanted, even going so far as having them bring animals, which were a rarity in the Seireitei into their home. His fascination with animals grew as he learned they were malleable. He could take them apart if they didn't struggle too much. It grew into a fascination with science as he learned that there was so much he could study and pull apart to discover the workings inside. But, in a way he missed the feeling of the warm flesh growing cool beneath his prying fingers. That was when he turned to human experimentation.
6. Cooking, to him, seemed something only fools would engage in. When the captain of the twelfth approached him with a tray of freshly baked sweets, Mayuri was certain that conjecture was true. But when Urahara took one of those sweets and calmly informed 'Mayu-chan' that altering reports wasn't to be tolerated, even if Mayu-chan did so well forging his signature, Mayuri wasn't quite as sure.
7. To Mayuri, his division can be divided into two sections, the useful and the garbage. For him, every member of the research division, especially his favored members like Hiyosu and Akon, were some of the few people he could associate with without feeling his IQ drop exponentially. As for the rest of the division, the members that had no talent or use as far as he was concerned were modified to suit his needs and disposed of when he had no further use of them.
8. Mayuri finds it amusing when people suggest that he let Nemu live her own life. You don't let your microwave go off on its own. Why would she be any different? She's just as much a tool as one.
9. Following Aizen's departure from the Soul Society, Mayuri was part of the team that created the technology to allow Hinamori to continue to gain nutrients and function while in her coma. While the case fascinated Mayuri, the amount of time it forces him to be with Unohana, who hasn't forgiven him for the mysterious disappearances of some of her division members, nearly drives him mad.
10. When he first encountered the quincy ryoka, Mayuri had no interest in him whatsoever. He was strong for his age, powerful too, yes. But, he was more distracted by the strange powers his female friend exhibited. After learning that the quincy had survived (something he was somewhat glad to discover, despite his initial anger), he began to plot how to retake him. The girl he could find any day. But this quincy, although he had studied many others and this one exhibited the same level of foolishness as his brethren, certainly met and surpassed every expectation of Mayuri's. He intends to see just how far he could stretch the boy if he ever encounters him again.
11. Mayuri's dearest hope is that somehow Urahara has died in exile. Some nights he wakes up in fear that his captain has returned. It's a sinking paranoia, but he keeps a close monitor on reports of suspicious behavior at the entrances to the Soul Society. He's terrified because no matter how many decades have passed, no matter how much praise he has received; he still believes he is inches away from surpassing his former captain.
12. He's often offended at the thought that others believe he might betray the Gotei-13 to join Aizen and his lackeys. Admittedly, it is tempting. He has only scarcely seen the powers of the hyogoku and longs to discover more about it. However, he finds the idea of licking Aizen's boot for a taste of power disgusting. He'd rather cling to the easy power of being a captain than lower himself to serve a former equal.
13. He strived to become a vice-captain under Urahara for years, working hard and making an effort to please the man. However, never once has he admitted to admiring him. Mayuri respected his intelligence, but he considered Urahara Kisuke to be one of the worst, least influential captains of the Gotei-13, being utterly ineffective and doing nothing but letting his genius rot with his lazy attitude. And was more than happy to take the division and research department into his own hands.
14. He will never forgive Kisuke for the fact that his greatest achievement is tainted by Kisuke's work. After hours of exciting designing Nemu, weeks of working out every little kink, he came across a page of notes of his captain's. And realized that every problem he had worked through, every thing he had done in the creation of the girl had been done by his captain in a project labeled 'Ururu'.
15. Mayuri first achieved ban kai seven years after Kisuke left his position as captain of the twelfth division. As the division wasn't as fighting oriented as the other divisions, he wasn't expected to perform as captain so much as fill the duties Kisuke left behind, but he worked at it anyways, for fear that if Urahara returned, he would be expected to return to his place as a vice-captain, something Mayuri refused to allow to happen to himself.
16. As a vice captain there were three people he hated more than anything in the world, his captain and the two women he often went to go drinking with. Many an early morning, he'd have to go searching for the man and have to deal with the two women and their... especially creative nicknames for himself.
17. The armor Mayuri wears is not a recent creation. In fact he designed it as a child and made additions to it, as he got older. He discovered it in his files when he was feeling unnaturally nostalgic. It amused him and fascinated him to bring that childhood vision to reality. The inhumanity of the mask he wore made his victims' fear all the more palpable.
18. Mayuri can't bring himself to be interested in the female form (or even the male form) in a sexual sense. He feels oddly disengaged when looking at people's bodies, yellow eyes finding every tiny imperfection or hint of one. Instead of desire, he feels a need to deconstruct them, to remake them to destroy the imperfection.
19. Mayuri has actually done a lot of work with other divisions, convincing other captains to allow their subordinates to join field tests of sorts. Byakuya has volunteered Renji for many of these, causing the redhead to purposely take a very long side-track to go from the sixth to the thirteenth division when visiting Rukia.
20. Unlike most of his division, Mayuri holds little to no fascination with the living world. While Hiyosu and his goons spend lunch hours camped in front of the transmission screens, Mayuri spends just as much time camped over the twitching form of an organism kept just alive enough to breath and pump blood. He doesn't disparage them of this. Really, he enjoys having them to perform the task, but there's a lingering taste of dislike when often their work turns out to be more beneficial to the whole of the Gotei-13.