For as long as there have been sentient beings, there was an interesting oddity about them. Each person, each sentient being, can hear the voice of their soulmate in their head, as long as the soulmate is alive, of course.
To humans, soulmates were the person whose personality would best match yours. It didn't necessarily have to be a romantic match, though most were. It could be anything, to them. Lovers, best friends, completely platonic, it didn't matter. To humans, soulmates were just fitting personalities. It could certainly get awkward at times, if they weren't romantically involved, which was why they usually were (arranged marriages and such notwithstanding).
To the angels, soulmates were a blessing. Not all of them had soulmates, so it was a rare and happy thing if they did have one. But, of course, it was always platonic and practically never romantic, because they're all about 'pure' and 'not defiling themselves and making themselves obscene in the eyes of the lord through sexual acts' like that. Weirdos.
To the grim reapers, soulmates were their partners. Each grim reaper paired up, with their soulmate, and did their job with them. They always worked together (though they may not necessarily be fond of one another- it was actually just as common to find a pair who despised each other as it was to find a pair who didn't), and they strengthened their abilities based on how close they were to the other.
The exception to all of these previous beings, however, were demons. To demons, soulmates are everything. Demons have to eat souls to survive, but souls are sort of a poison to them as well. It's a controlled suicide to save your own life situation, eating souls. Demons can't stand them, but have to eat them; it's the only way they can survive. When a demon finds their soulmate, however, they no longer have to eat souls to survive and easily manage to just live off the natural energy emitted by their soulmates. It's like having to eat your least favorite foods all your life because that's all you can eat, then meeting someone and being able to eat just your favorite types of food forever, if you wanted a human metaphor for it. Just about all demons have romantic relationships with their soulmates, mainly because it makes sense. Being around them allows them to function without having to devour souls, therefore, living with them and being near at practically all times was almost a no brainer.
Most times, a being's soulmate was the same type of being. There were times when a demon and an angel, or a demon and reaper, or angel and reaper, were soulmates, but never, in all of history, has one of these supernatural beings had a human soulmate.
It was absolutely unheard of, because soulmates were supposed to be able to protect each other, and even if it sounded bad, humans just weren't as strong as the other three types of beings. There had never once been a human- whatever soulmate bond.
However, just because it had never happened before, didn't mean it can't happen.
The very first human that had a soulmate who was one of these beings was known to the majority of England as Earl Ciel Phantomhive, known to the royal family and a select few others as The Queen's Guard Dog, rulers of the underworld.