IPA: /fʌlˈfɪlər/
KK: /fəlˈfɪlər/
A person or thing that completes or carries out a task or duty.
The order was processed by a dedicated fulfiller who ensured everything was correct.
Fulfiller → It is formed from "fulfill" (from Old English *fullfyllan*, meaning to fill completely) and "-er" (meaning a person who). A fulfiller is a person who completes or fills something fully.
Think of someone who 'fills completely' ('fulfill') — the '-er' just means a person. A fulfiller is someone who completes or fulfills a task.