Check/Counter/Sorta
Volcarona @ Leftovers
Ability: Storm Drain
EVs: 4 HP / 252 SpA / 252 Spe
Modest Nature
- Bug Buzz
- Fiery Dance
- Giga Drain
- Quiver Dance
I run this in my AAA team, and it's obnoxious to Keldeo. If we're talking Choice Specs Keldeo it can easily work as a hard counter, because if Keldeo is locked into a Water move it can't touch Volcarona. If it uses Secret Sword though, uh....
252 SpA Choice Specs Adaptability Keldeo Secret Sword vs. 4 HP / 0 Def Volcarona: 197-232 (63.1 - 74.3%) -- guaranteed 2HKO after Leftovers recovery
Yeeaaaah. Since Volcarona is slower this is obviously less than optimal for a switch-in, especially if the enemy has Stealth Rock up and you die instantly. Even running pure defense wall gets you...
252 SpA Choice Specs Adaptability Keldeo Secret Sword vs. 252 HP / 252 Def Volcarona: 142-168 (37.9 - 44.9%) -- guaranteed 2HKO after Stealth Rock and Leftovers recovery
Yaaay. To be fair, without Stealth Rock it becomes a 3HKO, which is more viable.
+1 252+ SpA Volcarona Giga Drain vs. 0 HP / 4 SpD Keldeo: 302-356 (93.4 - 110.2%) -- 56.3% chance to OHKO
On the other hand if Volcarona switches in from a KO and uses Giga Drain while Keldeo uses a Water move (I know, I know, it requires the opponent is either stupid or assuming you are stupid), then Keldeo has a better than 50% chance of instantly dying. Alternatively, you can Quiver Dance, they hit you with Water, they still stay in trying to Secret Sword or something, and definitely OHKO and jump up to full health even if Stealth Rock weakened you on the way in.
This was really honestly a bad, bad 'mon back when Aerilate wasn't banned and my usage of it was more a commentary on my preference for fun ideas over good ideas, but with Aerilate banned its biggest problem -that they'll just switch in in Dragonite and Extreme Speed you for a OHKO- no longer applies. Of course, it's second biggest problem -Stealth Rock- absolutely does apply so bleh.
Mostly it's reliant on the fact that I have never, even against high-Elo players, had anybody anticipate Storm Drain Volcarona unless they've fought me before and saw it then, so people pretty much always stick in and hit it with a Water move.
(I have no expectation this will be selected as a 'real' counter or check, but I've found this Volcarona useful for fighting Keldeo while still being useful for an array of other purposes, including messing with Poison Heal Suicune)