Slitherine

I received this AMERICAN HERO during this campaign but I don’t know what to do with that guy

All my artillery already have this feature "COUNTER BATTERY FIRE", so which unit to give KENNETH LEE ?

Put it on an M7 I guess and hope for the best if you happen to then place this unit on hills. This is one of the heroes that will rarely come useful without some extra help which is other heroes that add range, camouflage, or rely on favorable terrain placement. The biggest issue is, that even if you put him on a 15 cm piece, the enemy artillery will likely still have your gun in its range, and since AI decisions are based on prestige and damage dealt, your artillery will end up being the prime target for enemy guns anyway. I guess it is more useful in the early game where you have a lot of 7.5 cm pieces with one less range, but in 1944 the is not much the hero can achieve alone.