Keldeo shares its Water/Fighting type with only one other Pokemon, Poliwrath, but has much more over the muscular tadpole. It has a much higher offensive stat in its special attack stat, and can effectively run a mixed set without giving up power by using its signature move, Secret Sword, which comes off its above average base 129 Special Attack, but deals physical damage.
Ability
Justified: This is probably the biggest thing about Keldeo that I see is redundant. Keldeo only has a base 72 attack stat -- too low for it to abuse Justified, so it's next to useless.
Calm Mind
Item: Life Orb
Nature: Timid
EVs: 4 HP / 252 SpA / 252 Spe
- Hydro Pump
- Secret Sword
- Hidden Power Electric / Ice
- Calm Mind
Basic Offensive set that aims to sweep through unprepared teams. Hydro Pump is Keldeo's most Powerful move, being able to OHKO bulky Scizor after a Calm Mind and OHKO specially defensive Jirachi after 2 CMs. Secret Sword is what makes Keldeo even harder to switch into. It acts like a Fighting-typed Psyshock or Psystrike. For example, it guarantees a OHKO on Blissey and Chansey after a Calm Mind. Keldeo is one of the few Water-types that does not learn Ice Beam. Keldeo can get past this by using Hidden Power Ice. However, Hidden Power Electric is generally recommended, as it hits both flying types that Hidden Power Ice would hit, and opposing water types that wall Keldeo otherwise like Slowbro, Slowking, and Tentacruel.
Other Options
A Swords Dance set with Aqua Tail, Close Combat, and X-Scissor with Aqua Jet and Sacred Sword as other options is gimmicky at best, as Keldeo has only a base 72 Attack stat.
Work Up can allow Keldeo to hit with both Special and Physical attacks, but the loss in power due to splitting EVs is considerable.
Counters
Roserade is probably your best bet. It resists everything Keldeo commonly carries bar Hidden Power Ice. Venusaur is another option, but Keldeo is usually paired with rain, so Venusaur can't function as well as it needs sun to outspeed Keldeo. Slowking, although rare, can potentially wall Keldeo even better than Roserade, as the standard 252 HP / 56 SpD Slowking can survive even a +2 Hidden Power Ghost or Electric and KO the water horse with Psychic or Psyshock.