At some point in the past, Regigigas created the three Legendary titans to resemble its own image.
Prior to All About Arceus IX, Reg was awakened by Palmer with the help of Brandon's Legendary titans so that it could be used to stop a legendary crisis in Sinnoh in the case that there was one. Since it was not necessary at the time, Palmer released it back into Snowpoint Temple.
Reg first appeared at the Snowpoint Temple as a silhouette in Bogging Down Quagsire after Platinum, Maylene, and Candice ended up camping there after their defeat at the hands of Jupiter. It let loose a growling noise that startled Platinum, causing her to pull out a Poké Ball in case it was a wild Pokémon that she would need to catch for their safety; however, she fell unconscious from her injuries and the Poké Ball fell and rolled towards Regigigas, who went inside the Poké Ball.
In Double Trouble with Dialga and Palkia V, during the final battle with Cyrus on the Spear Pillar, Regigigas appeared from out of Platinum's bag to save Diamond and Moo from entering a black hole. After saving him, Regigigas decided to choose Diamond as its Trainer and the two go to battle against Dialga and Palkia. Reg managed to grab Dialga and Palkia and pin them down so that the black hole they created while fighting would not be able to widen any further. Although it was able to stop them briefly, Dialga and Palkia were encouraged by Cyrus to fight even harder. The power they expelled became too great for Reg to handle and it was sent flying and defeated. Later, it helped to carry several Gym Leaders to a hospital after the battle had ended.
In Unplugging Rotom (Heat, Wash, Mow, Fan, Frost), Diamond revealed that even though Reg was now his Pokémon, it refused to come out of its Poké Ball, and he enlisted the help of Dr. Footstep to see if he could read its footprints to figure out what is wrong with it. Soon after, a Giratina arrived from the Distortion World and begun wreaking havoc on Eterna City. Reg quickly let itself out from its Poké Ball to battle the Renegade Pokémon. However, it was soon forced to switch opponents when it was attacked by Charon's Heatran. The Lava Dome Pokémon proved to be a tough opponent with its strength and Fire-type moves, but Reg managed to deal heavy damage with its Crush Grip and nearly defeats it.
In The Final Dimensional Duel III, when Diamond was attacked and seemingly killed by a Shadow Force from Giratina after pushing Pearl out of the way of the attack, Reg suddenly turned around and appeared to leave. However, Reg quickly returned and revealed that it only left to bring the appliances to allow Rotom to change forms and attack Charon. When Giratina tried to attack it from behind, Reg simply tossed it on Heatran and throws a large pile of snow on the two. When Pearl traveled to the Distortion World, he held Reg's Poké Ball since Diamond had mysteriously disappeared beforehand.
In The Final Dimensional Duel IX, at the Distortion World, Charon sent Heatran out to attack Diamond and Pearl, causing Reg to come out of its Poké Ball once again to fight it a second time. As their battle continued, several other Legendary Pokémon joined the fight. During the battle, Reg, along with several others, were put to sleep by Darkrai's Dark Void; however, Shaymin managed to save them by turning their Abilities to Insomnia with Worry Seed, saving them from Giratina's Dragon Pulse. The battle with Giratina ended when it attempts to escape, leaving only the Pokémon that could fly to be able to chase it back into the real world. After Charon and Giratina were defeated, Reg, along with the other Legendary Pokémon, emerged from the Distortion World with Paka and Uji in its hands. It was then revealed to have chosen to stay as a part of Diamond's team.