emissary

When presidents can’t attend state funerals, they send an emissary to pay their respects. An emissary performs a specific job on someone else’s behalf.

Emissary is a word you hear often around governments, but anyone can be one or have one. Say, if you’re feeling too lazy to get off the couch you could appoint your little sister to be your emissary in fetching ice cream from the kitchen.