正体を

one's true identity

→ 正体
phrase Rare #34,266
one's true identity The particle を marks 正体 as the object of the verb, as in “reveal one's true identity.”
JA: 彼はついに正体を明かした。
EN: He finally revealed his true identity.

森じゅうにお前たちの正体を知らせてやる

I'll make sure the whole forest knows what you really are.

— もののけ姫 (1997)

あの女の正体を知っているか?

Do you know who that woman really is?

— さらば、わが愛/覇王別姫 (1993)

お母さまにあなたの正体を見せてあげよう

I’ll show your mother who you really are.

— ハウルの動く城 (2004)