Some one who is not good at keeping secrets. In my native language it is called "chugalkhor" but it's a slang. So I can't translate it.
What do you call such a person who can't keep secrets because his instincts don't let him.
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A slang term for a person who can't keep a secret is a blabbermouth.
Someone who talks a lot about other people, including about their personal information, is a gossip or gossiper (you can say it either way).
I thought of another one: loudmouth. A loudmouth can be someone who talks too loud, but they can also be someone who blurts out secrets without thinking.