Keldeo is a quadrupedal, colt-like Pokémon with a cream-colored body. It has a short, rounded snout with large nostrils and blue eyes. It has a large, feathery red mane, long, dark blue, ridged eyebrow-like protrusions, and a single cream-colored horn that curves slightly. The back of its head, as well as its neck, is covered with fluffy, light blue fur. Keldeo has blue hooves placed backwards, which are capable of ejecting water. Its tail is light blue with white spots and resembles an elaborate feather in a cap.
Long ago, a fire started in the Moor of Icirrus because of a war among people. In this fire, Keldeo was separated from its parents, but together, Cobalion, Terrakion, and Virizion took care of Keldeo. They acted as its parents, and they taught it knowledge and moves for survival. Keldeo eventually grew and even surpassed the three Pokémon. One day, Keldeo took off from the forest with no one knowing why. When the four met again, they taught Keldeo the move Secret Sword. Keldeo was trained by the other members of the Swords of Justice. In Generation V, Sacred Sword was one of its signature moves.
Keldeo tends to cross the world and often appears at beautiful watersides. Keldeo also travels in order to further improve itself through training. It can walk across the water by ejecting water from its hooves.
Keldeo has two forms: Ordinary Form and Resolute Form.
Upon learning the move Secret Sword, Keldeo transforms from its Ordinary Form into its Resolute Form. The dark blue protrusions on its head become shorter and lighter in color, now resembling ears. Its horn becomes dark blue and grows larger and ridged. On the side of its head are three feathers colored aqua blue, green, and orange.
Keldeo is at its full power when in Resolute Form. It is thought that when Keldeo becomes resolute, its body fills with power and it becomes swifter. Keldeo's jumping capabilities become fast to the point where it is hard to follow. The horn on its forehead is filled with hidden power. The horn can change into a sword that can cut through anything. It is said Keldeo must endure harsh battles in order for the forehead horn to develop, which then awakens its true power.
Keldeo debuted at the Pledge Grove, where it waited for Cobalion, Terrakion, and Virizion to arrive and teach it Sacred Sword. After acquiring the move, Keldeo follows its mentors to where they would confront the Shadow Triad who tried to capture the trio earlier, which was near the P2 Laboratory. There, the Shadow Triad are locked in combat with Cilan, Chili, and Cress, the Striaton Gym Leaders. It first appeared in The Shadow Triad.
Resolute Form Keldeo appears as a Poké Ball summon in both the 3DS and Wii U versions of the game. It slashes opponents with Secret Sword.
Keldeo trophy in Super Smash Bros. for 3DS
Keldeo also appears in Resolute Form as a trophy in both versions of the game.
NA: This Water/Fighting-type Pokémon is a lot like a multicolored unicorn. In its Resolute Form, its horn will grow longer and its vision is keener. It is this Form that appears on the battlefields of Smash Bros. Be wary of its Secret Sword attack—Keldeo's charge will send anyone hit flying.
PAL: The Mythical Pokémon Keldeo looks a lot like a multicoloured unicorn. In its Resolute Form, its horn grows longer and its vision improves. It can also use Secret Sword in that Form, and that's what it will do in this game. If Keldeo starts charging after you, watch out - that huge horn will send you flying!
Keldeo returns as a Pokémon summoned from the Poké Ball, retaining its behavior from the previous game. Keldeo also appears as a Spirit in both its Ordinary and Resolute Forms.
Pokémon Mystery Dungeon: Gates to Infinity: Keldeo is an important character in the storyline where he was a friend of Virizion, and a Magnagate savant. He was imprisoned in Glacier Palace, and there he wrote a letter to Virizion telling her that they weren't friends anymore. This letter caused Virizion to stop believing in friends. After being rescued by Umbreon and Espeon, he apologizes to Virizion for writing the letter. He joins the player's team after the player Pokémon returns to the human world.
This Pokémon was unavailable prior to Generation V.
Keldeo has two interchangeable forms: Ordinary Form and Resolute Form. These forms are mostly cosmetic. The Resolute Form was introduced in Pokémon Black 2 and White 2.
If Keldeo is taught Secret Sword, it will change into Resolute Form. If it forgets Secret Sword, it returns to Ordinary Form. Keldeo taught Secret Sword in Pokémon Black and White remain in Ordinary Form when traded or transferred to later games, though in Pokémon Sword and Shield, an Ordinary Form Keldeo that knows Secret Sword will change to Resolute Form if it is in the party at the end of a battle, or upon learning or forgetting any move other than Secret Sword.
Keldeo is most likely based on the Scottish legend of the kelpie, a supernatural water horse that sometimes appeared as a lost pony. This is evidenced by the fact that Keldeo appears in the Crown Tundra, an area based on Scotland. Keldeo is also likely based on the kirin, also referred to as the "Chinese unicorn" which are often depicted with a single backward curving horn and possess the ability to walk on water.
Story-wise, Keldeo draws inspiration from the character of d'Artagnan from the novel The Three Musketeers. Like d'Artagnan, it is the latest and youngest addition to the group. Keldeo's tail also resembles the D'Artagnan trademark feathered cap.
The three feathers on Resolute Keldeo's head may symbolize its three companions and teachers: blue for Cobalion, yellow for Terrakion, and green for Virizion.
Keldeo may be a combination of kelpie (a water horse from Celtic folklore), kelda (Old Norse for well, spring), and rodeo (a sport involving skills with horses, cows, and other livestock). It may also involve dio (Italian for god).