informant

noun2

1. •
   One who relays confidential information to someone, especially to the police; an informer.

   • One of her chief informants is Alicent’s handmaiden Talya (Alexis Rabin), whose inside info runs so deep that she’s the first to catch wind of Viserys’ death. — 2022
2. •
   A native speaker who acts as a linguistic reference for a language being studied. The informant demonstrates native pronunciation, provides grammaticality judgments regarding linguistic well-formedness, and may also explain cultural references and other important contextual information.

   • The informant learns his language by formal training and, more importantly, by constant exposure to its use. He cannot repeat to the linguist what he has never seen or heard. — 2003