Synonyms for Keen:

adj.

critical (adjective)
discriminating.
dry (adjective)
thirsty.
enthusiastic (adjective)
gung ho, zealous, avid, athirst, born-again, thirsty, interested, alert, ardent, agog, hooked, fervent, intense, impatient, intent, wholehearted, mad about something, warm, motivated, eager.
impressionable (adjective)
sensitive.
intelligent (adjective)
quick, sensitive, sharp, clever, penetrating, incisive, discriminating, bright.
knifelike (adjective)
knifelike.
raring (adjective)
raring.
sensitive (adjective)
observant, eager.
sharp (adjective)
spiny, needle-like, prickly, spear-like, pointed, stabbing, thorny, needle-point, acute, bristly, razor-sharp, cutting, razor-edged, jagged, barbed, spiky, pointy, spinous, lancinating, sagittal, sharp, spiked, Thistly.
sharp, piercing (adjective)
observant, cutting, extreme, acute, strong, penetrating, caustic, pointed, incisive, tart, perceptive, trenchant, razor-sharp, quick-witted.
worried (adjective)
solicitous.

n.

clever, sensation, perceive, quick-witted, quick, kinaesthetic, assail, perception, assault, smart, ability, sense, distinguish, bright, Sharp-witted, alert. meditative, deductive, sound, probing, thoughtful, reasoned, insightful, sensitive, trenchant, intellectual, careful. fantastical, wonderful, mewl, terrific, beep, splendid, clink, fantastic, glorious, dreamy, chime, tremendous, ripping, bleep, hot, divine, super, sensational, chink, marvelous, peep, click, fabulous, creak, superb. astute (noun)
clever, judicious, bright.
eager (noun)
interested, intent, enthusiastic, ardent.
intense (noun)
extreme, strong, biting.
keen (noun)
knifelike, great, intense, stabbing, good, swell, incisive, not bad, lancinate, neat, perceptive, cutting, lament, groovy, nifty, intelligent, discriminating, piercing, penetrative, sharp, bully, exquisite, smashing, acute, corking, lancinating, penetrating, peachy, slap-up, bang up, cracking, dandy.

v.

emotion (verb)
lament.

Other synonyms:

kinaesthetic, chime, athirst, probing, assail, Thirsting, mewl, trenchant, clink, urgent, perceive, serrated, agog, thoughtful, ripping, gung ho, observant, raring, distinguish, bleep, creak. longing, left-handed, sensation, Sharp-witted, perception, warm, Two-edged, desperate, assault, double-edged. marvelous, tremendous, beep, solicitous, razor-sharp, nuts. fabulous, dreamy, chink, bursting, alert, sense, peep, sensational, wonderful. glorious, click. fantastical. fantastic. acidulous
biting.
alert
quick.
bang-up
bang up.
cry
mewl.
eager
avid.
fanatical
rabid.
intense
strong.
Other relevant words:
lament, eager, peachy, nifty, biting, hot, perceptive, bully, piercing, exquisite, penetrating, bright, dandy, knifelike, quick, agog, smart, ripping, great, fervent, double-edged, ardent, avid, clever, strong, alert, swell, stabbing, zealous, incisive, slap-up, trenchant, enthusiastic.

Usage examples for keen

  1. As we all know, dogs have a keen sense of hearing. – The-Life-of-Me-an-autobiography by Johnson, Clarence Edgar
  2. " Is that you, Mr. Keen – The Tracer of Lost Persons by Robert W. Chambers
  3. Morty's an awfully good sort, and not keen on music. – Kildares of Storm by Eleanor Mercein Kelly