handler

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun One that handles or directs something or someone.
• noun One who trains or exhibits an animal, such as a dog.
• noun One who acts as the trainer or second of a boxer.

from The Century Dictionary.

• noun A person employed in the transfer or placing of things by hand, or in some special kind of manipulation or management: as, a freight-handler; a handler of dogs or of game-cocks; a handler of fish for propagation (used of one who selects the ripe fish from a catch).
• noun The first bath or pit in a tannery.
• noun In ceramics, a workman who attaches to the bodies of vessels the handles, which have previously been molded in plaster-of-Paris molds.

from the GNU version of the Collaborative International Dictionary of English.

• noun one who trains or exhibits animals.
• noun someone in charge of training an athlete (especially a boxer) or a team.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun literally One who handles something (especially manually) or someone.
• noun in combination A controller, trainer, someone who handles a specified thing, animal or person (especially a prizefighter), which can be specified in a combination
• noun computing A subroutine that handles a particular situation such as an event or exception.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun (sports) someone in charge of training an athlete or a team
• noun one who trains or exhibits animals
• noun an agent who handles something or someone