Jellicent plain fails against SubCM Keldeo with Toxic Spikes up too, by the way.
And yes, Latias as close to being a perfect counter without having to run something which is utter fodder as you can get. You can easily fit her onto offensive teams as a supporter sweeper if you need to (just watch out for Tyranitar and all that stuff, stuff you should be watching out for already). I'm going to be planning to revamp Latias when Keldeo comes out, and it would be a bit like my
DW Latias analysis, if you want to see how you counter Keldeo with it.
As I've said before, I don't think Keldeo would be broken. Top tier OU, sure, but not broken. It has counters/checks that can fit on all sorts of teams and does not restrict team-building that much at all. There's at least ten Pokemon in OU that can semi-counter it already (as seen earlier in the thread). Looking into lower tiers, we also have
Roserade, which can OHKO with Leaf Storm even when Keldeo is at +1 while setting up Toxic Spikes to severely cut Keldeo's sweep time.
Slowking is another Pokemon which I would call a perfect counter to Keldeo, since it resists both of its STABs, has great SpD to shrug of Hydro Pump and Hidden Power Ghost, has reliable recovery in Slack Off, Regenerator, and can either put Keldeo out of commission with Psyshock or just Dragon Tail it away. It's a pity that he's such Volt-turn prey, though, but I guess we can't have everything. Also, Keldeo in general hates sun teams, especially sun teams with Venusaur in them.