• English / United States: Keldeo
• Japanese / Japan: ケルディオ
• Japanese (Romanized) / Japan: Keldeo
• Japanese (TL) / Japan: Keldeo

Keldeo is a young and naive Pokémon that was training with the Sacred Swordsman Pokémon Virizion, Terrakion and Cobalion so that it could become a member of the Sacred Swordsman. It got impatient waiting for their approval to fight Kyruem, the strongest Dragon Pokémon in the world to prove that it was worthy to become a Sacred Swordsman. Keldeo is able to speak to humans.

Keldeo went to challenge Kyurem on its own despite not being able to use its sword yet and against the council of the Sacred Swordsman. During the battle against Kyurem, Keldeo learned that it was still too weak to face Kyurem and it got its horn broken off and it fled because it was scared. Virizion, Terrakion and Cobalion came to rescue Keldeo but they were frozen in ice by Kyurem.

After falling on the train that Ash Ketchum and his friends were traveling on, it befriended them and with their encouragement, Keldeo decided to face Kyurem once more. During that battle, Keldeo learned to use its sword properly and was able to prove to Kyurem that it was a strong Pokémon after all. After Keldeo proved to be worthy to become a Sacred Swordsman, Virizion, Terrakion and Cobalion had no objections and agreed that Keldeo should join them.