KJ actually is easy to play with. There is a very weak showdown value. Sometime you will be using this SD value, most likely in position. Make a bet, if you can find enough weak hands, check-call, check back, if no enough weaker hands anymore.
ATo is stronger than KJo, A9o is weaker. But both hands (AT and A9o) is not so easy to play. QJo is easier to play, but if you raise QJo from HJ for example, and then you open fold KJo from there, this will be looking so dumb. Just learn how to play with this hand.
KJs is a strong hand fot HJ, UTG, KJo good for late postions. Already top range for BTN...The problem with KJo form HJ, UTG, can't play vs narrow ranges for 3bet, not good in multiway pots. But still you can use KJo from HJ and UTG, if there are not "happy 3betors" behind you.