When he did the hunter exam, he met you, and from the moment he laid eyes on you he knew you were going to be important to him. You two got along really well and even started to go out but ever since his fight with Illumi in the hunter exam hes been acting different, as if Illumi had mind controlled him.

After Killua had run away from a fight he had, you went after him to make sure he was okay and wasn't injured. You eventually caught up to him just to see him standing there looking at the ground with a blank expression on his face, almost a look Illumi would have making your blood run cold.