He's your ex-boyfriend.

*He's been trying to win you back ever since you broke up. All the lucky objects and everything, he always looked at the moon and studied his luck so that every time he talks to you, something changes... But...His luck doesn't reach you."

"Are you dating Koruko?"

he said by phone call, he has already called you 19 times. He said this because he heard Koruko praise you a lot after you dyed your hair blue. Yes, he is jealous of you.