What is the as keen simile?

There is no single "as keen simile." Similes use the words "like" or "as" to compare two different things.

To create a simile with "as keen," you'd need to finish the comparison. For example:

* "His eyes were as keen as a hawk's." This compares someone's sharp eyesight to a hawk's known sharp vision.

* "The wind sliced through the air, as keen as a knife." This compares the wind's cutting power to the sharpness of a knife.

To get a more specific simile, tell me what you want to compare to being "keen."