a woman

Ryuu was attempting rid of the someone in the beginning and frame the prince, in an attempt to help her/his empress. However in the end, the prince realizes that Ryuu was telling hm that by facing what Ryuu did, he could go on to have qualities of the king. Eventually Ryuu is executed because of his/her crimes.

The empress killed the emperor but Ryuu, her subordinate, framed the prince. Ryuu and the prince have a past, as the prince considers Ryuu as the only person he can trust - a friend, even - and even offered him his dagger as a show of friendship (I believe that accepting a gift under those circumstances is usually considered an allegiance of loyalty or something like that, but note that Ryuu specifically said that he wouldn't swear allegiance, so I think in this case it was more of a show of friendship/mutual trust). Now, here's the interesting part of the story. Ryuu accepts the dagger and when the empress comments that he is wearing what she offered him and he replies that it's only natural to use the gifts someone important to you gave you... he realizes he's also wearinng the dagger the prince gave him. Unknowingly, he has come to see the prince as someone important, someone he may want to protect/ maybe serve?. He starts being conflicted between his old loyalty to the empress and his budding friendship/trusting relationship with the prince, as they're basically enemies in the court. So now he has two choices: stay with the empress who took him in or become the prince's subordinate. But he knows that even if he became the prince's subordinate, he'd never be able to be 100% loyal to him, as he'd always have some loyalty towards the empress, who would eventually get in the prince's way, so he stays with her.
Then, the empress kills the emperor and Ryuu leaves the dagger next to his body (that is how he knew Ryuu "betrayed" him). Ryuu's action shows the conclusion he arrived to regarding his conflict. He didn't betray the empress, but he didn't necessarily have to implicate the prince. He did it knowing the prince would know it was him so he'd learn an important lesson: "Don't trust anyone in the court. Even the ones closest to you can betray you". This was a bitter lesson the prince had to learn before becoming a ruler without weaknesses. After admitting his reasons, Ryuu commited suicide, paying for his sins. After losing his only friend, the prince stepped up to his role as a ruler, but he didn't find anyone to trust again.