noun
betrayer; traitor; turncoat; informer
A person who betrays trust, a group, or a cause. Covers everything from personal betrayal to treason and informing on others.
彼は仲間を警察に売った裏切り者だ。
He is a traitor who sold out his comrades to the police.
裏切り者は絶対に許さない。
I will never forgive a betrayer.