Let me preface this by stating I really do hope
Karen is not as potent as she seems.
A single Supporter Card is not enough to make the most popular//effective META Deck available to date unviable...
Sure it is; it all depends upon what that Supporter does, so a general statement that is clearly false. You may not believe
Karen is that Supporter, but that still doesn't make your statement true. It seems a foolhardy claim in a format that contains
Battle Compressor and
VS Seeker, especially with several competitive decks already using one or the other or even both. Decks with both could easily include a single copy of any deck-killer Supporter so that you could quickly send the Supporter from deck to discard to hand, at which point each
VS Seeker can turn itself into that Supporter.
Again, even if you end up being right about
Karen, the premise is flawed; a hypothetical future "deck killer" could be released, and it may even already have been released but because it predated the release (or at least discovery) of a deck, that deck never became a thing.
Lysandre might be an example: though I think it is ultimately good for the format (I've seen the tyranny of a too secure Bench), imagine what its lack would mean for certain decks!
I'd like to think that they would put an end to Night March eventually (and that is coming from a fellow Night Marcher) but Karen seems to only be a simple beginning. It will take a lot more to completely stop Night March from doing what it does. Even with an Anti Meta Deck starring Karen....you still would need to guarantee you can do at least one of the following:
I want to be clear about how
Karen works and make sure you realize why it is such a problem for Night March. I've seen good Night March decks. I've seen bad Night March decks. I've seen good and bad players with either. Even though I often run it myself on the PTCGO, I may even be one of those bad players and/or have a bad build. Some of my opens, I only have to use a couple of my various deck thinning cards; in those cases
Karen would be a problem only in that instead of now being able to thin my deck of spare resources (improving future draws) or dig deep for things like another
Double Colorless Energy, I'll need hit those same deck thinning cards again so that I can set up again.
Against the average open, Night March loses valuable turns as it tries to restart itself. That means losing Prizes going both ways, as it both fails to take Prizes it needed to take and while giving up Prizes as its small Pokémon are easily OHKO'd. You'll still have some of the setup cards you need available, but one-third to two-thirds have already been used, and if you whiff on them, you whiff on having a decent attack. Night March doesn't mind its glass cannons being OHKO'd because it streams them and takes out targets that are more Prize or resource rich. Whiff on the KO more than once or twice and you probably have no hope of recovering, at least without luck/opponent misplays.
Then there is a "bad opening", though right now as long as you do still get your field set up it would be seen as solid (perhaps even ideal!): you end up using almost all of your deck thinning or hand discarding cards in the first few turns. Right now this is great as you're only left with the stuff you really, really want in your deck. After
Karen, once she resets everything, you have to wait for your opponent to KO stuff in order to get it into the discard pile; you most likely cannot spare those Prizes so its game over!
I suspect you referenced an "anti-metagame deck" to give yourself the least favorable odds for this scenario, which I actually applaud. In a discussion like this, that is the correct approach. Except you were mistaken;
Karen is expected to be so potent because it needs no such thing! Instead, take
any deck and unless it needs to keep certain Pokémon constantly in the discard pile,
Karen could work in that deck. What is more,
Karen serves a significant role besides countering Night March. If it already was dedicating at least a single slot to
Sacred Ash,
Karen replaces
Sacred Ash. If they have two
Super Rod, or
Super Rod plus something else to reclaim Energy cards, then
Karen replaces a
Super Rod.
So take all the current Night March match ups; however you rate them, they are going to be significantly worse in a metagame with
Karen in the card pool, because
Karen will be played.
1) Go Second: Not only is this out of your control but depending on which Anti Meta you run, you could be eliminating the chance to get your Pokemon evolved to put the "item block" in place. Night March has proven to us time and time again to be almost too effective, especially if that Player chooses to use turn 2 as his/her 1st turn. Yeah, you might kill that Joltik...but doesn't that really just end up hurting you in the grand scheme of things?
It doesn't matter whether you go first or second, unless your opponent is running Night March backed by
Vileplume.
This is actually part of why I don't expect Vespiquen/Vileplume decks to take a huge hit; they just need a spare attacker to buy time until they can use
Professor Sycamore with a hand full of Pokémon to get
Vespiquen hitting for at least decent damage again. The big point is that
Karen works after the fact; it doesn't need to be used on your first or second turn. In fact it is quite the opposite; you want to use it mid-to-late game!
Whether Night March or its opponent takes an early lead,
Karen forces your opponent to struggle to get Night Marchers back into the discard pile, as the Night March deck now lacks all the cards used to aid in that set up the first time. It isn't just whatever is in your discard pile either; you'll most likely have less copies and less room for
Shaymin-EX on your Bench. Night March wins on the margins, at least for competitive decks. Your opponent can't keep up in Prizes because you don't just set up and take them fast, you use Basics worth a single Prize that trade well either either Evolutions or Pokémon-EX. Not every deck loses to Night March by a large Prize margin; many are just a Prize or two off from their own victory.
2) Rhythmic Karen: Please understand while I say all of this, that I am extremely excited for this card. It is the first (but slightly weak) step of better things to come! Hopefully, Pokemon will do more things like Karen to put an end to all the Marching. HOWEVER, unless you plan on running a deck that is STRICTLY about putting Night March down I hope you come prepared with your luck. Karen is going to have to be like a Guardian Angel as far as draw timing, and in comparing to the other SU cards in your hand that you now won't be able to use that turn (This includes the ole' Seeker trick as well). Sure, it could save your Active... but with this card coming out Night Marcher's aren't going to stand still. Fates Collide just gave us Mew, Glaceon EX, and made N Standard again. Not too mention a few other cards that I personally don't use, but nonetheless might help Night March in some way or another...Bottom line, Good luck Karen. We may have to keep Bunnelby around after all.
We don't need additional Night March counters. It isn't the deck so many try to make it out to be; most seem to mistake its popularity - partially due to
Shaymin-EX (ROS) being the only really expensive card in it - as a sign it is overpowered. Well it is overpowered, but not compared to the rest of the competitive metagame. Frankly it seems like one of the fairer deals; even when it is one-sided, at least it is fast so you can try to actually get best two of three happening.
To counter Night March once
Karen is released, you don't need an entire deck dedicated to it. You just need a deck that doesn't clash with
Karen itself and a single copy. Yes, with no additional support it requires some good timing. "Good timing" is merely "Any time after Night March has burned through resources to set up." Night March doesn't need to be totally useless to lose its place in competitive play... it just needs something like this that takes its across the board match ups and lowers pretty much all of them.
N makes
Karen more dangerous to Night March, not less; they cannot be used in the same turn (barring a deck no one uses) but your opponent can throw the 8+ Night March Pokémon back into your deck, and if you were winning, you must set up the next turn or else expect
N to drop you to three, two, or even one card. Next to no hand, no thinned deck, no strong attack. As for getting back things like
Battle Compressor to set back up, a turn attacking with
Bunnelby is a turn you're not taking a Prize. If you use
Puzzle of Time or
Bunnelby for such purpose you are
not using them for their usual purpose of reclaiming
Double Colorless Energy cards. That is a very significant opportunity cost.
Edit: I ended up revising the whole thing. Didn't change the meaning, just cleaned up phrases, typos, etc. Like how some uses of "whiff" somehow ended up saying "wish" instead. XD