Hey I'm new to the game but I was wondering if both players can recruit and attack on each others turns and if a plot twist is played from your resource row does it stay in play?
And how do you ko a character because I'm still confuse after reading the rulebook.

Yes, but only during their steps (each player has a recruit step and an attack step).

yes it stays in play.

there is a seperate KO'd zone. cards in this zone have no
influence on the game. it might be easier to think of it as a "discard" pile.

You KO characters, either with a direct effect, like "Finishing Move" or Jean Grey (I forget which level)'s ability, or else by stunning more than one character. At the recovery phase the owner must choose one of his characters to ready, and the others are automatically KO'ed.

As Mitchell_W pointed out, KO means place in your discard pile.