One who hearkens; a person who listens attentively or pays heed.
From 'hearken' (Old English 'heorcnian') plus '-er', creating an agent noun for one who performs attentive listening.
'Hearkener' is almost never used because 'listener' is shorter and more natural, yet it perfectly shows English's productive pattern of creating agent nouns—this word exists in dictionaries but rarely in actual speech, making it a 'ghost word' of sorts!
Complete word intelligence in one call. Free tier — 50 lookups/day.