Quill coat is great vs camps. So for the counterjungling part, you either need ward coverage, to counter his moves (eg, if u see him go for ur wraiths, u go for his), or u need to ping teammates, when u see him. Or alternatively, do as I do, and ask yourself: "Which route would this champion usually start?" And then go the opposite, cuz if u think "oh, I'm Amumu/Nautilus, I should start Blue" then the enemy Lee sin sure as hell is thinking the same, and he is already considering whether he should just go for a quick red-steal after his blue, or if he should go for first-blood

Quill coat is an improvement however slight it may be.

something you may want to try out that a couple high elo lee sin players are doing is this build path (on lee obviously):
Ancient Golem -> merc treads -> tiamat -> warmogs -> blue sightstone -> spirit visage. then obviously complete your other items. This build provides extreme tankiness, and the amount of self regeneration you have in a fight is ridiculous. This build has like 4k hp flat, but you can regain about 5k in a fight giving you an effective 9k hp.
Otherwise, quill coat is just a pickup on a jungler you want to build as a tank. Out of the top 3 junglers, its only viable on lee (especially because of the free ward).

Most characters I've seen are based of people, yes.