The Guardian System
Each member of the royal family, upon their birth, are given a guardian. Guardians are people, especially chosen even before their birth, to protect the royal family in any way possible. The sole reason their souls even exist is to do this job. After they reach an appropriate age they are taken away from their families to be specially trained in how to use their natural abilities and powers; fight, defend, heal and various other things they will need to know to protect the member of royalty they are destined to serve. Upon the birth of their master they are sent to the castle, their new home, and take their vows to always protect the one they serve. At that time or sometime very soon after they create a contract with their master in which they gain a similar royal insignia as the royal family which can be used to identify them as guardians and they gain a sort of telepathic link with them that enables them to sense where their master is if something goes wrong. It is said that the bond that is formed between guardian and master is very strong. When their master dies, it is said that the guardian will die as well; or at least a member of royalty should not die if their guardian is still alive to protect them. After making the contract, a guardians body no longer ages in order to keep them at their physical peak so that they can do their job. Guardians also have a similar powerful crystal that if destroyed will strip away their powers and their insignia, therefore taking away any sort of link they have with the one they are supposed to protect. Guardians also must obey whatever their member of royalty tells them to do unless it conflicts with their safety and their guardians ability to protect them. Normally a member of the royal family is matched with one guardian of the same gender to protect them. It is incredibly rare that a Princess or Prince to have more than one guardian or to have a guardian of the opposite gender.
The Princess' Guardians
Unlike most members of the royal family, the Princess at birth was given five guardians - all of them male, which is completely unheard of. This however, set her up to be believed as the Princess who was told of long ago in a prophecy; that a princess who appears with five male guardians would be set to be the most powerful in the world and the one to save humanity. But, because of this, her parents, especially her father, the King, is very protective of her and does not let her enjoy herself as she pleases. This, as well as his overprotectiveness, also made him add an extra vow to their ceremony - a vow that prohibits them from getting to close or forming any sort of romantic or physical relationship with his daughter, which would be looked down upon anyhow as guardians, though of very high status in the castle are not normally of noble birth and so are not allowed to ever marry into royalty. To this he added that if any of them did they would be imprisoned, exiled, or worse. This is what makes any relationship between her and Yami (or any of her guardians) so difficult and extremely taboo. The contract between the Princess and her guardians took full effect around the time she was three years old. She formed the bond between them by shyly and hesitantly giving each of them a sealing kiss as tradition stated (the only time this would be allowed and the only time, as yet, she has been able to use her powers.)