Wiktionary
main man
n. (context informal idiomatic chiefly African American Vernacular English English) One's closest male friend.
Wikipedia
Main Man
"Main Man" is a rock song by American singer-actress Cher from her eighteenth studio album, Cher. It was written and produced by Desmond Child. It was released as a promotional single from the album in 1988 by Geffen.
It had originally been recorded by Desmond Child & Rouge in 1978.
Usage examples of "main man".
The other was Jed Griffin, the President's main man for breaking logjams.
Readers call Weber similar things, but mostly they call the Baen offices several times a week demanding more from their main man.