handler

(literally) One who handles something (especially manually) or someone.

(in combination) A controller, trainer, someone who handles a specified thing, animal or person (especially a prizefighter).

(derogatory) An advisor or manager to a person occupying a position or office to which the speaker believes the holder does not possess the typical qualifications and/or experience.

(programming) A subroutine that handles a particular situation such as an event or exception.