For "keen" meaning enthusiastic, eager, or interested:
* Apathetic: lacking interest, enthusiasm, or concern.
* Indifferent: having no particular interest or concern.
* Uninterested: not interested.
* Reluctant: unwilling to do something.
* Unenthusiastic: lacking enthusiasm.
* Disinterested: not interested in something or not having an opinion.
For "keen" meaning sharp, intense, or strong:
* Blunt: not sharp or pointed.
* Dull: lacking sharpness or intensity.
* Weak: lacking strength or intensity.
* Mild: not strong or intense.
* Unfocused: lacking sharpness or clarity.
For "keen" meaning sharp, perceptive, or intelligent:
* Obtuse: slow to understand or lacking in intelligence.
* Stupid: lacking intelligence.
* Naive: lacking experience, wisdom, or judgment.
* Unperceptive: lacking the ability to notice or understand things.
The best antonym will depend on the specific meaning of "keen" in your context.