ANGELS ARE THE BOSSES OF SHINIGAMI
-We do know that there are angels in the anime, but in manga they still haven't appeared, and it's highly unlikely that they won't appear. The main theory I present to you is that angels are the bosses of Shinigami. Shinigami used to be humans, and now they have to reap souls in order to be forgiven for killing themselves when they were humans. Basically anyone who has killed themselves becomes a grim-reaper after death. At least that's all that we know. {Manga-wise} Another thing that it's frequently mentioned are the supervisors {the bosses of Shinigami} who provide the Shinigami with the list of who's going to die, and are the ones responsible for creating the Shinigami.
-Now the theories I'm going to discuss are these: angels are these supervisors; angels have foresight powers; Queen Victoria may be part-angel; and so Vincent Phantomhive. There's a lot to unpack so let's get started.
*Rewatching the Shinigami OVA*
The scheduled death was at 4 p.m., and William tried to reap the writer's soul prematurely. The supervisors not only knew when the writer was going to die{exactly at 4:00 p.m} but also knew how he was going to die. If you pay attention, Grell and Will reaped the soul at exactly 4, despite Will trying to reap it a little earlier. How come they knew much information{the supervisors}? The answer is : either they are the ones who decide such things or they have predictive abilities.
SUPERVISORS ARE THE ANGELS
I believe the ones who talked with Will and Grell in the OVA about the exams are not the supervisors we are talking about, but rather the teachers, like Will and Grell were in the beginning of the OVA. These supervisors are mentioned frequently in the manga.
In Abrahamic religions, souls are collected by angels of death or angel of death called Azrael. Now I'm not saying that Azrael will make an appearance in the manga, I'm just saying that it's not far-fetched at all to assume that angels are the one supervising the Shinigami.
Shinigami used to be humans and they were brought to life by higher-beings only to make them collect souls. It's evident that angels created those Shinigami, whose duty is simply to judge and retrieve the souls to those higher-ups.
THE POWER OF FORESIGHT OF ANGELS
Angels of death have immense powers, and they know when and how somebody is going to die. Sometimes they don't know all the details, and that's why they send Shinigami to investigate. They knew that a demon was going to kill many people, but didn't know which devil.
I'd like to mention the Red Butler arc. The reason why the higher-ups didn't know at first that Grell was helping a human kill those women was because the cause of death was murder by a human. Only when Grell impulsively kills Madam Red, they find out about it. That's why they send Will to pick up Grell. It's highly likely that Will appearing just when Grell was about to get killed by Sebastian WAS a coincidence.
They'd know if Grell was on the death list.
Who writes those lists but one with the power of foresight. Angels do know when and how people are going to die. Unless, of course, a demon or a Shinigami interferes. But, the future is not constant as well.
QUEEN IS PART ANGEL
Before we get into this theory, I'd like to talk first about Queen Victoria. If Princess Charlotte hadn't died from child-birth, Queen Victoria wouldn't be Queen or she maybe wouldn't even exist at all. After the death of Princess Charlotte, there was a succession crisis where her father and the other brothers were pressured to have children. Victoria's mother was the sister of Princess Charlotte's husband. And Victoria's father was the 4th son of the king. Victoria had a lonely childhood. Her mother was extremely overprotective and she couldn't talk to other children. She could only do things that her mother and Sir John Conroy would approve.
{This is a self-portait}
Victoria hated Sir John Conroy, and he's rumored to not only be her mother's lover but also Victoria's biological father. Her mother wanted Conroy to be Victoria's private secretary, but Victoria detested the man.
In 1835, Victoria had a severe fever. Could she be that she died, but her life was spared by the Shinigami? Or was it sooner? Now let's talk about the manga Queen Victoria.
Brothers were rushing to have babies and were competing with each other on who can produce the heir. There were rumours that Queen Victoria was not royal. So what if Victoria's real father was actually an angel or a Shinigami? And that might mean that even Prince Albert was part-Angel or Shinigami. I have so many questions Ms Toboso. So many questions.
VINCENT IS PART SHINIGAMI
I can't post anymore photos here, but I'll try to explain this theory. In many flashbacks and even at the special chapter, Vincent has been shown to know about the future and was able to even to predict his death. You might say that he knew that he was in danger, but I think that this has to do with the Shinigami blood in his veins; the same reason why Ciel can see the Shinigami. Yeah, I think Undertaker fell in love with Claudia and had an affair with her.
CONCLUSION
I think that Undertaker saw at the recording of Claudia's memories a big secret of the Royal family. Queen Victoria isn't the real Queen. She's half-angel and she's manipulated by the Angels who are the higher-ups of Shinigami. I think (and many others!) John Brown is a Shinigami or Angel. It's possible that he's actually Prince Albert.
