the.com/responder

means A person or system that answers a call for help, especially in an emergency, and is often classified by how fast they arrive — 'first responder' being the one who gets there before anyone else.

from Built from 'respond,' which comes through French from Latin 'respondere' — 're-' meaning 'back' and 'spondere' meaning 'to pledge or promise.' To respond was originally to make a return pledge, like answering a vow with a vow. Over centuries it loosened from formal promise-swearing into simple answering, and 'responder' emerged as the noun for one who answers — later specializing, in the 20th century, into the person or unit who answers emergencies.