誰の味方

whose side

→ 誰
phrase Rare #40,738
whose side Used to ask which person or group someone supports, often in a confrontational or suspicious way. 誰の味方なんだ is a casual form meaning “whose side are you on?”
JA: 彼は誰の味方なのか、まだ分からない。
EN: I still don't know whose side he's on.

誰の味方せん

I’m not taking anyone’s side...

— ロード・オブ・ザ・リング/二つの塔 (2002)

あなたは誰の味方なの

Whose side are you on?

私は誰の味方もありません

I'm not on anyone's side.

誰の味方なんだ

Whose side are you on?

— いまを生きる (1989)