Just defeated the the four kings, now who's this kaathe dude and what should I answer to his question, yes or no?

So you beat the Four Kings before placing the Lordvessel? That took some guts .

If you answer to Kaathe, Frampt no longer will appear on Firelink Shrine and you'll gain the "evil" ending, because he'll be the one transporting you to the place where you deposit the Lordvessel to enter the final area (when you've got all four lord souls). Also, he'll give you access to the Darkwraith covenant too.

Ah damn looks like I'm goin to get the evil ending then! I've joined the darkwraith, is there anything cool I can get from this?

you decide the ending at the end of the game, the choice with darkwraith wont push ya towards anything. =)

the thing you get out of darkwraith is the endlessly useable invade stone

Yes but I answered "yes" to his question, and now frampt has gone from the firelink chamber. 

Also how can I acquire Humanity? I want to offer him 30 humanity for the darksword and dark armor set.

Double post

Sad news I need 3 (or 2 if crystal weaponry isn't titanite) titanite slabs and I'm at the end of my 2nd playthrough and I already wasted the one from the stray demon, what to do?

I can't express me hatred for New Londo in words. Those ghosts are just UGH!

Did you pursue Siegmeyer's of Catalina quest line to the end? He gives you a Titanite Slab. 

Also, you can farm them out of the Darkwraiths in New Londo, but it's a very rare drop. It's better if you go there with a Covetous Gold Snake Ring and 30 Humanity (for a 410 Item Discovery rate). If you have a Symbol of Avarice, even better.

I f***** hate super-smough. The little bastard seems to like only buttslaming his entire way through the battle. Of course, I can only burn through my estus flasks (the few that remain after taking care of ornstein) while the lightning shockwave takes bites out of my health.