handler

noun

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

  "The handler of a weapon gets a symbolic sensation of deadly power."

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

  "The spy's handler told him to approach the subject by posing as a dog handler."

• 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.
• A subroutine that handles a particular situation such as an event or exception.