Curse.
She was bound.
She shouldn't be in this world, but due to some strange combination of circumstances, she ended up at this time. Little did he know that this was a conspiracy from the beginning, a huge network.
He was a killer and the future head of the family who beat up enemies. He met her during the hunter exam and was attracted by her.
This is what is called "love at first sight".
"If I disappear, you must take good care of yourself." She said in her heart countless times, moving away from him at a speed he didn't know.
Because I love him.
"Let's play a game. If you can find her, I will return her to you." The man smiled at him.
He went anyway.
Just because he still owes her a promise.
A promise to never separate.