yea, tomikichi wear straw hats, always give up an item when you kill them (careful -- a few of them activate the item when they drop it), and carry a sword. only 1 per level, every level.
to get every achievement you have to complete each mission on each difficulty and also earn a ninja 5 on each difficulty. you can't just play through hard and get easy and normal, too.
tomikichi, on the other hand, only need to be killed once per level. so, if you kill him on easy, you don't have to kill him on normal or hard.