Canary is a servant to the Zoldyck family. She may not realize it but she is Killua's friend, but thinks that their difference in status means that they can not be friends.
Gon asked her why she had Killua's skateboard. She answered the first time that it was a gift. Later, she tells us he had dropped it.
She has a flash back about the tiny skull in her staff. He tries to to give it to her but she refuses it. Then, he tosses it and it lands at her feet and he runs off.