Keldeo is a quadruped, 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, which are capable of ejecting water. Its tail is light blue with white spots and resembles an elaborate feather in a cap.
Due to being in possession of the move Sacred Sword, Mikayla can have Keldeo transform into its Resolute Form at will. When transformed, the dark blue protrusions become shorter and lighter in color, 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; similar to the colors of its comrades.
Keldeo cares immensely for its trainer, and shows the same aloof confidence displayed by Mikayla. Like Jason's Aipom and Nathan's Emolga, it isn't usually stored within its Pokéball, deciding instead to remain at Mikayla's side. There are a slight few exceptions to this, however.
Keldeo's ability, like that of its teammates, is Justified; an Ability which allows it to increase its Attack power dramatically if a Dark-type attack is used on it. It can also alternate between its Ordinary Form and its Resolute Form at will. Doing so dramatically increases its speed and allows it to use different moves.