The Clinical Characteristics of Love
1. She doesn't cringe when she looks at his real face which is refreshing. Nemu is the only other one who doesn't and he knows that's only because he removed her disgust response.
2. Once, he asked her if she was willing to let him dissect her. Unlike most people, her reply was not an emphatic 'no'. She asked him why he was so interested in a quick dissection when he could study her for as long as he wanted.
3. She knows that contrary to popular belief, there is nothing wrong with Kurotsuchi Mayuri's sanity. However, sometimes she suspects there might be a problem with hers.
4. Their first date is fairly typical. They have a lively discussion about his latest article and he asks her to edit his next one.
5. It takes three months for Isane to stop shuddering every time their relationship is even mentioned. She doesn't think that the poor girl will stop twitching any time soon though. (She had told her not to disturb her though.)
6. The first fight they have is over Nemu, but not for the reasons most people would assume. She wants him to teach her about the concept of privacy but he doesn't understand the problem. It's nothing the girl hasn't seen before anyway. She wins the argument eventually but still locks the doors. (Nemu wasn't the only interruption, just the most persistent.)
7. Once Shunsui figures it out, it only takes two hours before everyone else knows as well. Reactions range from horror to pity. Some of the braver members of her division even ask her if he's done anything to her. His division, on the other hand, secretly bet on how long it'll take for her to end up on his autopsy table. (A somewhat morbid sense of humour is required for survival in the Twelfth.)
8. The first sign their relationship might have an effect on him is when a member of the Twelfth is resting peacefully in one of her division's beds, rather than knocked out on a table in his private lab.
9. People suspect his behaviour in Hueco Mundo is a result of her influence as well but aren't stupid enough to say that anywhere near him.
10. He would never admit it but even he occasionally finds her smile unsettling. She has never considered him unnerving though.
11. He saw her in battle once. He has never been a poetic sort of man but there is something about the clinically precise way she cuts down her opponents with the minimal amount of effort that he finds almost beautiful.
12. She enjoys walking with him in the gardens of her division and discussing the various plants found there (all medicinal, of course). His habit of pointing out their less benign uses is slightly irritating though.
13. It is possible that Nemu's first actual love interest may owe his life to her intervention.
14. Being the closest thing Isane has to a mother-figure, she takes it upon herself to be seen in the vicinity of her dates, preferably accompanied by Mayuri. Anybody not scared off is suitable dating material. (Nobody's managed it yet.)
15. She knows that most people have no idea what on earth she could possibly see in him. (The answer is a rare chance to have an intelligent conversation.)
16. He respects her as a woman of science even if she doesn't always agree with his methods.
17. Unfortunately, Ashisogi Jizou and Minazuki do not get along as well as their wielders. They have opposing natures after all but they gradually form an uneasy truce.
18. They are possibly the least romantic people in Soul Society yet their relationship seems to work somehow.
19. Certain brave people have considered signing a petition against them having children. (It would never get past Yamamoto though. He doesn't want to think what Unohana would do to him if it did.)
20. They do eventually have a son. Nemu is his idol and he spends most of his childhood being babysat by her. He doesn't really take after either parent but often terrifies people by accidentally appearing to. (Only his parents realise that it may not be such an accident after all.)
This was inspired by a conversation with x nihilo about their story Match Me which featured gen Mayuri/Isane.