Keen is a related term of thorough.
In obsolete|lang=en terms the difference between keen and thorough
is that
keen is (obsolete) brave, courageous; bold, audacious while
thorough is (obsolete) through.
As adjectives the difference between keen and thorough
is that
keen is showing a quick and ardent willingness or responsiveness, enthusiastic, eager; interested, intense while
thorough is painstaking and careful not to miss or omit any detail.
As nouns the difference between keen and thorough
is that
keen is a prolonged wail for a deceased person while
thorough is (uk|dialect) a furrow between two ridges, to drain off the surface water.
As a verb keen
is (rare) to sharpen; to make cold or
keen can be to utter a keen.
As a preposition thorough is
(obsolete) through.