Arc 1, New Moon
A person with compatible bodily fluids.
Just what in the world was that supposed to mean? 
The next day, Ernst once again returned to the library. The head butler threw significant glances at Ernst, who secluded himself in the library every day. It seemed like he knew what was contained within the library.
Ernst disregarded the head butler’s glance with a single cough. He opened the door to the library.
The pleasant smell of paper wafted over and enveloped him. The overwhelming majority of books were treasure troves of knowledge. They had comforted and guided Ernst ever since he had been young.
Ernst went to browse the book spines. He wanted to verify the words that the man had murmured.
First, he drew out a dictionary.
The dictionary contained here was thin and not very likely to be useful, but it might also shine a faint light that could lead the way.
Those words fell from Ernst’s mouth without him meaning to say them.
A person with compatible bodily fluids.
Partners of different races whose bodily fluids are perfectly compatible with each other. The probability of a person with compatible bodily fluids being born is one in a million, and the probability of being able to encounter throughout an entire lifetime is one in several tens of millions – was what the dictionary said.
It was something that was so extremely rare that it was uncertain whether it actually existed. Just what kind of circumstances signified that one’s bodily fluids were perfectly compatible? Ernst wanted to find this out, but the dictionary didn’t go into further detail.
Ernst stared at the bookshelf, then let out a breath and reached for the suspicious book spine. He wasn’t very sure, but the answer was probably written somewhere inside here.
After a large amount of books once again fell to the ground, Ernst glanced inside one of them.
“Drowning in Your Fountain” 
So, they drowned in a fountain. They drowned in a fountain, so just what were they doing. Working hard to maintain his composure, Ernst let out a curse at this book.
He took a deep breath and opened the book.
He had… found the answer. That is, if what was written in the book was correct.
People with bodily fluid compatibility were, in sum, like what was written in the dictionary. The sweat, saliva, blood, and also semen of both parties all felt desirable to the other. Their sweat, the way they tasted – all of it matched the other person’s preferences.
The man had said that he had been told that he smelled like an animal. Ernst couldn’t believe it. The odor of the man’s body was always the scent of a forest overflowing with life. It was the scent of fresh grass in the spring. It was flowers just beginning to bloom, the warm scent of life.
Just as always, Ernst sat on the man’s lap in front of the fireplace. Ernst brought his nose to the nape of the man’s thick neck, filled his chest with the man’s scent, and lapped at him with his tongue the same way the man did to him last night. A fresh and vibrant taste spread out over his tongue.
Though Ernst heard the man’s flustered voice, he ignored it and continued.
“I understand what you meant when you mentioned our bodily fluid compatibility. Have you ever encountered a person like this before?”
Ernst settled his hips on the man’s thick thighs and looked up at him.
“I haven’t. Something like bodily fluid compatibility is… how do I say this, it’s, half a legend in that sort of world.”
“What do you mean, that sort of world?”
“In other words, that is, um, the kind of shop where you could buy a woman, or a man.”
Ernst knew that there were stores such as this. They showed up in the books.
“…Have you purchased from there, too?”
Ernst himself was self-conscious over how insecure his eyes became.
“Uh, um… Well, that is… when there weren’t any other options…”
After staring intensely at him for a long time, Ernst dropped his gaze. He gripped the man’s clothes with both of his hands.
So, just like those men that had appeared in those books, there were times where he had bought a woman. So, he also had feverish nights where he couldn’t help but have to purchase someone in order to let it out.
Ernst, who had sunk into silence, heard the man’s questioning voice. For the first time, Ernst felt irritated with this man who didn’t understand Ernst’s feelings.
 Google translated this line as “What the hell is that?”, which was my exact reaction to reading ‘bodily fluid compatibility’.
 These scandalous book titles are all innuendo. This book’s title is 『溺れる泉』(Oboreru Izumi), and the word 溺れる (oboreru) can mean both ‘to drown in’ and ‘to indulge in’. Ernst, obviously, thinks it’s about drowning.