IPA: /fərˈɡɛtə/
KK: /fərˈɡɛtər/
A person who tends to forget things or does not remember them.
She is such a forgetter that she often misplaces her keys.
Forgetter → It is formed from "forget" (from Old English *forgietan*, meaning to lose the memory of) and "-er" (meaning a person who). A forgetter is a person who loses memory or fails to remember something.
Think of someone who 'loses memory' ('forget') — and the '-er' just means a person. A forgetter is someone who forgets.