Used for a person or entity that corresponds to another in a different organization, system, or context; similar to a contact person or equivalent role.
See also: 対応
Regarding your inquiry, the person in charge will get back to you.
I'm corresponding by email with my counterpart at the overseas branch.
Compound of 対応 (たいおう, 'correspondence, handling') + 者 (しゃ, 'person'). The meaning follows straightforwardly from its components.