1.
to reveal one's true colors; to show one's real nature
For someone's hidden or disguised true nature, identity, or intentions to become apparent. Often implies the revealed truth is negative.
ついに正体を現した。
They finally revealed their true colors.
正体 means true identity or true nature. This expression is often used when someone who has been pretending finally shows who they really are. In fiction, it can literally mean unmasking a disguised character. The intransitive 正体が現れる (one's true nature comes out) is also used. Related: 本性を現す (to show one's true character), 化けの皮が剥がれる (to have one's mask ripped off).