In the case of akali you have to beat her and shut her down before 6. The best option in my experiene is make sure you are buying a pink ward on your first back. This way you can continue to damage her with targeted abilities if she tries to stealth harass. Remember to stay out of AA range so she can't proc her Q damage.
When you get back to lane push out HARD and place the pink in an area where she can't see it. At the very least make sure she won't notice it when placed. Harass her as best you can, and then kill her early. If you've pinked you'll also have stronger ganks from the jungle. By 6 you NEED to be ahead, this mean NOT playing passively.
I hate hearing when someone suggests playing passively on champions who aren't supposed to be passive. Would you rather be behind all game, or take the chance to be ahead of someone that has you countered through mid and late game?
Akali is a strong ganker. Post 6 you need to be ganking when she is mia. When she is lane, gank then too. Pretty much post 6 avoid her as much as possible, and the only time you should be engaging her is if you have a team to back you up. Hopefully, if you've set her back during the laning phase you can counter gank her. Follow her when she ganks, this can be easy with proper warding.
Originally Posted by Damein Sandow
To be completely honest the only way someone could counter someone is if. there was 10 types of champs, labeled type 1, 2, 3, 4, 5, 6, 7, 8, 9,10. each type did double damage to another type and received half the damage from it, and it created a perfect circle. of rock paper scissors but with 10. heck it could be 3, it could be as large as the pool of champions. each champion could have 1 other champion they do double damage to and recieve half the damage from. it would work as long as it is a perfect circle which it don't matter how many champs exist it always can be a perfect circle.
that is the only way for there to be counter play, pokemon had it, but pokemon's was not a perfect circle. some types like bug were weak to alot of things but not strong against alot of things, whilst psychic was not weak to alot of things.
until then any champ won't hard counter another. unless it is like malphite countering AD, but even then blackcleavers counter him.
As far as what damien has suggested, everyone is in some way "counterable". Just because A isn't dealing double damage to B doesn't mean they haven't countered a person. Some champions have strong kits that will make life difficult. You're Teemo, and you only have a haste as an escape mechanism. Burst, gap-closers, and CC counter him. You are Akali, Stealth detection, MR, HP, AoE damage and again CC. The list goes on... Champions have weaknesses in their kits, what you need to learn is how to exploit them.
In many situations people suggest passive play, sitting back, and letting your enemy decide how the lane will be played. Sit at your tower and farm, hug the turret (all this does is let the enemy free farm and have complete control of the match). In the end the enemy has free roam to gank any time they like, leaves your team weaker to fight objectives, and makes you afraid to even leave your turret.
Get in the enemies face, but play smart. Bait them as best you can. Play like you are WEAK and lull them into believing you aren't a threat.
The best example in all of this I can give is playing mid as Annie. She is a very balanced and easy to learn champion. She, like many other mages have a hard time against Kass. What do you do? He is going to poke all day with Q and zone you out till 6. All I do? Buy an early null mantle. The early magic resist will give you the chance to trade. He wants to harass? Harass him right back. Any time he goes in, you come in harder. The worst thing you can do is sit idly by and let them farm and deny, while you sit like a scared child at your turret. In the end you need to study up on how to counter the enemy, and how to out play someone should you get countered.
play smart, know your enemy, and DON'T play passively
(That's not to say be SUPER SUPER aggressive, just play your game and not your opponents. Don't let the enemy control the lane and you'll be fine.)