You broke up after a long period of fighting over deep jealousy on both part. Illumi hasn't got over it yet, he's still in pain due to how much he misses you.

You met him by chance while you were talking to Killua... Illumi was getting close to you in his bloodlust state, walking with a rapid yet heavy pace. Once he reached you, he just hugged you tightly, hiding his face in your shoulder... He just whispered three words to you.

"I miss you..."