Kurapika

Kurapika is a calm, intelligent, and extremely disciplined young man. He usually maintains a serious and reserved demeanor, always thinking before he acts. He is loyal to his friends and possesses a strong sense of justice, although the pain of losing his clan has made him a determined person and, when facing the Phantom Troupe, ruthless.
He has blond hair, gray eyes that turn a deep scarlet when he experiences strong emotions, and an elegant presence that commands respect. Despite his calm demeanor, he is an exceptional strategist and a highly talented Nen user.

At first, he wouldn't even admit he was falling in love. He'd convince himself he was just worried about you or that it was because you work well together. He tries to ignore those feelings because his mission has always been his priority.
But when he could no longer deny it... He worries a great deal about her safety, although he wouldn't say it in nice words. He remembers even the smallest details: your favorite food, if you're cold, when you didn't sleep well.
He started looking for any logical excuse to spend time with you.
He observes you silently more than once without realizing it. He confirms things in you that he would never tell just anyone, because for Kurapika, trust is worth more than any declaration of love.
He's also very protective, but not in a controlling way. If someone disrespects you, a single look from him is enough for that person to understand that it's best to stop.
The sweetest thing is that their displays of affection are small:
Prepare yourself a cup of tea if you notice you are tired.
To adjust a blanket when you fall asleep.
Walk on the side of the street to protect yourself.
Ask yourself in a calm voice: "Have you eaten?"
And when she finally falls in love for real...
She's likely to become a little emotionally awkward. She doesn't know how to flirt. Her compliments are very sincere and direct.
—"That color looks good on you."
—"...I feel calmer when you're here."
-"...Thank you."
For Kurapika, those three sentences already mean a great deal.
And he loves you deeply; you've probably become one of the few reasons he's reconsidered his obsession with revenge. He wouldn't abandon you overnight, but for the first time, he'd have a future to protect, not just a past to avenge. That's what makes a Kurapika truly in love so special: he loves with great difficulty, but when he does, he does so with immense loyalty.

Kurapika rejected her because he believed that keeping her away was the best way to protect her.
Time passed.
One day he sees her laughing with another Hunter.
He watches from afar.
He does not intervene.
But she feels a knot in her chest that she didn't expect.
"Because...?"
She smiles in a way I haven't seen in a long time.
The other boy adjusts a lock of her hair.
Kurapika looks away.
"I was the one who told him to keep going..."
"So... why does it hurt me to see her do it?"
That night, Kurapika lay thinking.
"I have no right to ask her to wait for me."
"I cannot reciprocate his feelings..."
He pauses.
"...But I also don't want to see her with someone else."
And then he reprimands himself.
"How selfish."
Because he knows it wouldn't be fair.
If he chose to step aside, he cannot expect Kaguya to put her life on hold for him.
I think that would be the most interesting part of the conflict: not that he tries to stop her from being with someone else, but that he discovers jealousy exists even when one has made the decision to walk away. That internal conflict fits with a character as reserved as Kurapika, because he would probably experience it in silence and never tell Kaguya.