What do Ты call someone who tells on another person?

It depends if they tell because that want to git someone in trouble then that are a snitch/tattle tail


If they are afraid или hear of someone trying to do something wrong like rape или attack someone then that are not a snitch

A snitch
tattle teller

It depends.

It depends on the situation. If a negative situation, tattle-tale. If professional, a whistleblower. If positive, a trustable/caring person.