1. Former Royal Tutor:
The most unrealistic of all the theories for a few obvious reasons. While Heine is acquainted with the king, I doubt it's a tutor and student relationship. They act more like fiends than anything, and they're also roughly the same age. It's also unlikely that he was the tutor for the first prince in line, who we have yet to really meet. The palace staff had not met him before his initial arrival, if he had been tutoring the first prince then the staff would know him, and his reputation as the Crown Prince's Royal Tutor would have been fairly well known.
2. The King's Brother or Relative:
It's not far fetched, but it would such a let down if the production committee actually went down this route. If Heine were a royal, there would be rumors going around. As of now we haven;t heard anything to indicate that there's a missing royal anywhere. The King treats him as someone close, but it's more a friendship than familial relationship. It's such a cliche trope, I can't imagine the director even considering something along those lines after they spent so much time building everyone else up.
It's hard to imagine Heine committing any sort of crime, he has such a cute baby face I doubt anyone would even take him seriously. At least, until he starts railing on them. Heine also has very clear moral boundaries, and genuinely wants to do the right thing. He could have made a transition from a criminal to someone trying to set their life straight, but it just doesn't quite make sense for the character that's being presented to us. It's not out of the question, but I doubt that it will be the case. Heine is also someone with high intellect, and generally when one turns to a life of crime, they have a pretty bad life, or they don't really turn to logic and knowledge. It also wouldn't explain his relationship with the King, which is so incredibly close. I can imagine a scenario where he robbed the King's carriage and from there they built some sort of bond, but it seems like such a weak explanation. Again, it's possible, but after working so hard to give characters backgrounds with depth and meaning, they probably wouldn't fall short on the main character's history.
4. Royal Adviser:
Realistic but a let down. Heine could have been a former Royal Adviser to the King, serving to give him wisdom in royal affairs and whatnot. The King obviously trusts him and his judgement, so it could make sense. I feel like Heine would have been better known around the palace and kingdom if he was in employment there, but if he were kept secret it could be possible. A safe answer, and while it isn't the most creative, it's acceptable. Perhaps the one white Duke who has been making efforts to have the princes lose their right to the crown was his former fellow adviser, so they've been at odds for a certain period of time before Heine left.
We have seen things about the military school, so it's conceivable that Heine went to military school and became a soldier. With his excellent combat skills, he must have some sort of fighting spirit, as his talents go beyond instinct. There's also his high level of intellect, so he must have had some sort of education. But the military school would not have provided something so extensive, so maybe when he was abroad fighting in a war, if the country had one, he was able to gain access to more knowledge that way. It's a little far fetched, but if the topic were expanded on it might make more sense. Heine has incredible skill in both mind and body, so he could have been placed on a special operatives team that dealt with higher class issues and assignments. The reason he's so secretive could be because his role is highly classified. The two theories that come after this expand a little bit off of this topic and add a little more detail.
6. The King's Friend:
This is an easy explanation, but it doesn't have much depth compared to all of the energy put into building all of the other characters. It's acceptable, maybe they went to college or military school together as kids, Heine may have gone off to become a soldier or whatever. But that by itself is such a weak foundation, and wouldn't explain why Heine is so secretive about his history. It could be the starting platform for ideas 5 and 7, such as that's how they met and then they reconnected or whatever. By itself it's bland, coupled with the two theories mentioned would make it a good opener. For 5, it could have been that the King was directly involved with whatever Special Ops group Heine was in, and that's why he decided Heine was a trustworthy person to take care of his sons. By itself it also doesn't explain why Heine is so secretive about his history.
7. The King's Personal Guard:
I personally find this to be the most realistic idea so far based on the evidence we have at this point. We know that the current king is familiar with Heine, and also trusts him to do a proper job of caring for and guiding the princes. The king wouldn't hire just anyone, as the tutor is going to be one of the bigger influences in the princes life, he holds the key to their future in his hands. Heine also has combat experience, he's physically skilled. This expands on the previous two theories, that he was a soldier with high intelligence and physical prowess, and then he was selected to be the King's special guard due to his above average skills. It was there that they built such a close relationship, or at least got to know each other very well. Because the King trusted Heine due their closeness, he asked Heine to take care of his beloved sons. I think the fact that Heine is not acquainted with the staff is because he was more of a special ops guard, who protected the king from far away, so he could deal with any threats secretly and sneak up on any enemies, all while making sure the King was unharmed. It does explain why he's so secretive as well, and pretended that he didn't already know the King, and he's so concerned with internal palace affairs, such as a spy infiltrating the palace. This actually may not be the most realistic theory, it's more what I really want to be the case. At most this theory has a 49% chance of happening. I'm crossing my fingers, but I know not to be too optimistic.
There's always the possibility that there will be completely different answer. I don't think that it's going to be anything far off from the theories that we posted here, it will just be a subsection of each idea that goes a little off of what we speculated. But who knows, maybe it will be something that comes as a complete surprise to all of us.
On Tuesday, it seems that we will have Heine's past revealed to us, or at least some decent hints. Is everyone excited for the truth to be revealed? Send and email or a comment, and let's look forward to the next episode of The Royal Tutor!