Cobalion congratulating Keldeo on mastering the sword
Kelden debuted in The Shadow Triad. It was first seen at the Pledge Grove, waiting for its masters, Cobalion, Terrakion, and Virizion. Once they arrived, they tested Keldeo to see if it was worthy of joining their group. Keldeo tried to cut the boulder with its sword, only to lose concentration and mess up. Terrakion and Virizion laughed off the failed attempt and helped Keldeo practice so it could try again. After finishing the training, Keldeo successfully mastered its sword and added a new cut to the boulder. Afterward, Cobalion had the others follow it.
In Something Suspicious, Keldeo and its masters arrived at the P2 Laboratory, where they found the Shadow Triad in battle with the Striaton CityGym Leaders. When Keldeo asked which side was evil, Cobalion stated that battles between humans had no moral alignments. Sensing evil from the Shadow Triad, Keldeo stepped in to attack them with its sword, easily pushing back their Pokémon. The Shadow Triad retaliated once Keldeo let its guard down, forcing its masters to step in and protect it. Cobalion praised Keldeo's use of its sword, but informed that it order to obtain another sword, it needed to be resolute. Cobalion, Terrakion, and Virizion used their swords to send both the Shadow Triad and the Striaton Gym Leaders flying. As Keldeo and its masters left, the Shadow Triad had their secret weapon, Genesect, fire its Techno Blast on them. Keldeo's masters stepped in front of the blast to protect their student, but were frozen and fell into the sea.
Two years later, in Pokédex Lecture, Blake and Whitley went to the Pledge Grove to fill their Pokédexes with data. After accidentally being summoned by Whitley's Foongy's Sweet Scent, Keldeo attacked the two to drive them away. Later that night, Keldeo was approached by Blake, who offered to help Keldeo strengthen itself and acquire new techniques. Agreeing to help, Keldeo joined Blake's team and was nicknamed Kelden.
In Therian Forme III, Blake sent out Kelden to chase Colress and the Plasma Frigate. When Blake tried to arrest Ghetsis, the Shadow Triad appeared to defend their master. Recognizing them from their previous encounter, Keldeo wildly attacked the Shadow Triad, distracting Blake enough for the Plasma Frigate to take off. Blake was forced to recall Kelden after managing to sneak aboard the ship. After realizing why it was angry, Blake promised to help Kelden get revenge on the Shadow Triad after they arrest Ghetsis.
In Flying Ship, Kelden battled Team Plasma on the Plasma Frigate. After defeating some Grunts, Kelden destroyed the chamber containing Kyurem to prevent Colress from abusing its power. To Blake's shock, the chamber was empty, but before he could process anything, Kyurem appeared and attacked the group. Kelden pushed Blake and Whitley out of the way, but ended up frozen in the process. Colress appeared and revealed that their efforts were in vain because the Plasma Frigate had already absorbed enough energy to freeze all of Unova. Colress then had Kyurem freeze Blake and Whitley alongside Kelden and had them thrown into the ocean.
Using an Aspear Berry given to it, Kelden broke free of the ice and took Blake and Whitley to the Abyssal Ruins. In PS542, Blake awakened and discovered that he could understand Kelden's thoughts, which he attributed to a special effect of the ruins. Kelden took Blake to the frozen bodies of its masters, but revealed it was unable to break the ice. With a Fire-type Techno Blast, Blake had Genesect melt the ice and free Kelden's masters. Afterward, Blake and Whitley rode on Kelden's back as they and Kelden's masters escaped the Abyssal Ruins.
Personality and characteristics
Kelden is a very kind Pokémon, willing to help others and strives to protect humans and Pokémon alike. It is very close to its three mentors, and has learned many things from them. Kelden learned the power of charging from Terrakion, speed and sharpness from Virizion, and how to keep a strong heart and to never flinch from an opponent from Cobalion. It is a bit clumsy, frequently tripping when not paying attention. Kelden always strives to be stronger, working hard to reach the strength of its mentors.