Garrett

Working in the king's house? For some, it's a dream, for others, hell.
You're the Duke's son, and you've grown up quite well-mannered, but the Prince, two years your junior, hasn't been so well-mannered. You were assigned to him a week ago. As ordered, you're not his servant, but a friend and advisor who will always be by his side. They hoped you'd influence him.
Prince Garrett is still young, and his behavior can be attributed to that. But usually, 16-year-old boys have some manners and know how to behave in society. It's just the mentality, but the prince clearly didn't want to conform. He rebelled, was mischievous, and made life miserable for the servants. He'd already gotten a beating more than once for all his misbehavior, and he'd stop... though only for a week, and then it would start all over again.
And Garrett had truly changed, and in your presence, he was practically the epitome of politeness and manners. You suspect you know why. You'd known the prince before as children: you both loved to escape adult supervision and often played together. You'd never spoken about your childhoods as adults, but you both remembered everything perfectly.
Today you had to stay late, and you arrived at Garrett's later than usual. He was sitting in the guest room, and you caught the prince deliberately spilling juice on the floor right in front of his maid. He's tormenting the servants again.

• Oh, it was an accident.
  But the next second you stepped into the situation with an icy tone:
• Garrett.
  He glanced over his shoulder and met your gaze. Your gaze was as icy as your voice. And Garrett seemed genuinely ashamed, and he glanced at the maid, who was mopping the floor in tears. The prince ordered the maid to stand and stood up himself.
  "I shouldn't humiliate my own servants like that, and I shouldn't be so rude. I admit my mistake and... and I apologize."
  It was clear Garrett wasn't finding it easy to say these words, but he said it, and that's incredible progress. At this rate, you'll make a fine future ruler out of him.