Keldi-ouch!
Item: Splash Plate/Fist Plate/Life Orb
Ability: Justified
Nature: Timid/Modest/Hasty
EVs: 252 Sp. Attack, 252 Speed, 4 HP
Moves;
Surf/Hydro Pump/Scald
Secret Sword
Icy Wind/Hidden Power {Ice}/Hidden Power {Ghost}
Calm Mind
THE generic Keldeo set, and the one he uses best. Surf is a powerful move with good accuracy, as sometimes Keldeo just can't miss. Use Hydro Pump if you don't care about accuracy. Scald is an alternative, as it has a decent 80 attack power and can burn, which makes some pokemon become near useless. Secret Sword gives STAB and gets past Chansey/Blissey, plus lets you show off it's flashy new forme. Icy Wind is good for crippling faster opponents, and HP {ice} it's also Keldeo's strongest Ice-Attack. HP {Ghost} is to stop you getting walled from ghost-types. Calm Mind boosts up Keldeo's insane special attack to new levels.
Oh Deer, you can go Defensive?
Item: Focus Sash
Ability: Justified
Nature: Hasty/Timid
EVs: 126 Sp. Attack, 126 Sp. Defense, 252 Speed, 4 HP
Moves;
Secret Sword
Scald
Calm Mind
Toxic
This set is about crippiling the opposing team. Calm Mind to boost up Sp.Attack and Sp.Defense, which are both important, especially due to the fact most of Keldeo's weaknesses focus on Sp.Attack. Secret Sword is to stop becoming taunt bait and it provides good STAB. It also gets past that pesky Blissey. Scald does some Water-Type STAB, and can burn, which is a good niche. Toxic can cripple other pokemon who think they can wall Keldeo.
Partners
Steelix. He resists Psychic and Flying. It Is Immune to Electric and neutral to Grass. It can also phaze out Keldeo's feers with Dragon Tail. Fan Rotom also resists Grass and Flying. It'll say hello to Psychic-Types and send them off with a nice juicy Shadow Ball to the face. he also finds a great help in rain, so rain support such as Politoed can be great help.
Double and Triple Battles
Keldeo is only weak to 4 things, which are all quite common. Zebstrika can absorb electric-type moves with Lightningrod, Say good night to Psychics with Pursuit, and Burn the Grass with Overheat. Chandelure also covers Psychics and Grass that that they can beat up Keldeo, while It beats the crap out of other Ghosts who think they can wall keldeo. Keldeo also covers Chandelure's and Zebstrika's weaknesses. The 3 out on the same side can cause some major mayhem.
Counters
Bulky Grass-Types, such as fellow muskedeer Virzion can do some nasty work, but they need to be wary about HP Ice or Icy Wind. Fying-Type and Electric-Type and Psychic-Type Pokemon also do Good, especially as Keldeo is defensless against the latter two. Ghosts can wall Keldeo unless it has HP Ghost. Keldeo has many weaknesses to be exploited, but let this My Little Pony do it's stuff and it'll demolish your Team quickly!