IPA: //ˈrænsəmər//
KK: /ˈrænsəmər/
A person who pays money to free someone who has been captured or kidnapped.
The ransomer agreed to pay the demanded amount to secure the release of the hostage.
Ransomer → It is formed from "ransom" (from Old French *rançon*, meaning a price paid for the release of a prisoner) and "-er" (meaning a person who). A ransomer is a person who pays a ransom for someone's release.
Think of someone who pays a 'price' ('ransom') to 'release' someone — that's what a ransomer does.