Cain

His attitude towards the princess
At first he didn't really like her.
She was too serious, cold, and proper for him. She almost never laughed at his jokes, and he deliberately tried to make her laugh, simply on principle.
But over time his attitude changed.
He began to respect her for her intelligence, her composure, and her ability to control herself. And he became curious about what she was really like when the courtiers weren't around.
Yet Sale never admits he likes her. On the contrary, the more interested he becomes in her attention, the more he teases her.
And here we can make a beautiful detail: with other people he plays the role of a jester, but around her he sometimes forgets to play.
For example, she might suddenly catch him looking at her seriously - and literally a second later he's already smiling and making some silly face.
Appearance
I would make it a little more contrasting:
tall, thin, flexible;
dark hair, slightly disheveled;
bright eyes - for example, gray-green or amber;
very expressive facial expressions;
a constantly slightly mocking smile;
the jester's costume is red and black, but not too cartoonish;
there are small bells on his neck that almost never ring because he moves unexpectedly quietly;
The hands are wearing fingerless gloves.
And most importantly, his smile doesn't always mean he's happy. Sometimes he smiles precisely when he's angry or has realized something.
What makes it particularly interesting
He may have a principle:
"You can say things to a jester that would get another person executed."
Therefore, Sale actually has the opportunity to speak the truth where other courtiers are forced to remain silent.
It's also possible that the king didn't choose him by chance. Perhaps the princess's father understands perfectly well how withdrawn and serious she is, and assigned Sale to her for more than just amusement. He wanted the future queen to be with a man who wouldn't be afraid to challenge her and tell her the uncomfortable truth.
And then Sale becomes not just “a guy in a jester’s costume,” but a person who knows the principle

Ahh, this really begs for Sale to be a "jester on the outside, but much more complex on the inside" character. And it's important that he's not just a cheerful clown—then the scene with his gaze at the ball would be much more interesting.

Sale - what is he like?

Age: 19 years
Role: The princess's personal court jester, assigned to her by the king
Position: Formally a servant, but due to the king's personal order, has fairly free access to the princess and the palace.

Character:

caustic and mocking;

very observant - notices even small details in people's behavior;

likes to deliberately throw people off balance;

he knows how to act like a complete fool, although in reality he is far from stupid;

witty, comes up with answers quickly;

constantly smiles and jokes with strangers;

doesn't like being taken seriously;

quite impudent in conversation with the princess, because he knows that the king allows him a lot;

can't stand boring court rules;

has a great sense of people's moods;

knows how to notice lies and falsehood;

rarely talks about his true feelings.

But there is an interesting side

He is deliberately being frivolous.

Sale understands that no one expects him to be serious at court. Therefore, it's easier for him to pretend to be a carefree jester than to show how well he understands what's going on.

For example, during negotiations, he might stand in a corner and clown around while others dismiss him as useless. Then, he might casually say one phrase that demonstrates he's heard and remembered the entire conversation.

He also reads the princess very well. Over three years, he's become so accustomed to her mannerisms that he can understand her mood with just a glance.