person inside a cartoon body suit
Refers to the performer wearing a mascot or character costume, especially at theme parks or events. Often used in fan discussions about who is inside a famous character suit.
I wonder who the person inside Mickey at Disneyland is.
voice actor (of a particular character)
By extension from the costume sense, used for the voice actor behind an animated character, especially in anime and game fan communities.
Apparently the voice actor for the main character of this anime is a newcomer.
insider; person on the inside
Slang for someone with inside knowledge or connections, often used in corporate, political, or online community contexts. Carries a slightly secretive or conspiratorial tone.
According to an insider, there's going to be a big announcement next month.
Literally 'person inside'. Originated from fan discussions about who is inside mascot costumes, then extended to voice actors and insiders.