What is he to you?

What is he to you? 너 생각은 어때? 무슨관계야? 

It has kind of double meanin. "What is your opinion about something? or What is your relationship with something?.  However, what is your relationship with something' is more common.

e.g)What is jazz to you? What is her relationship with you? 

A: I really hate John.

B: He's not that bad. 

A: What? What is he to you?

B: Well, actually he's my sister's boyfriend.

A: John 정말 싫어.

B: 걔 그렇게 나쁘지 않은데.

A: 뭐라구? 걔랑 무슨관계야?

B: 그게, 사실 내 동생 남자친구야.