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 falls unconscious from her injuries and the Poké Ball falls towards Reg, who goes 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 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 than it already had. Although it was able to stop them briefly, Dialga and Palkia were encouraged by Cyrus to fight even harder. The power they expel became too great for Reg to handle and it was sent flying and defeated. Later, it was used 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 is now his Pokémon, it refuses 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 arrives from the Distortion World and begins wreaking havoc on Eterna City. Reg quickly lets itself out from its Poké Ball to battle the Renegade Pokémon. However, Reg is soon forced to switch opponents when it is attacked by Heatran. The Lava Dome Pokémon proves to be a tough opponent with its strength and Fire-type moves, but Reg manages to deal heavy damage with its Crush Grip and nearly defeats it.
In The Final Dimensional Duel III, when Diamond is attacked and seemingly killed by a Shadow Force from Giratina after pushing Pearl out of the way of the attack, Reg suddenly turns around and appears to leave. However, Reg quickly returns and reveals that it only left to bring the appliances to allow Rotom to change forms and attack Charon. When Giratina tries to attack it from behind, Reg simply tosses it on Heatran and throws a large pile of snow on the two. When Pearl travels to the Distortion World, he holds Reg's Poké Ball since Diamond had mysteriously disappeared beforehand.
In The Final Dimensional Duel IX, at the Distortion World, Charon sends Heatran out to attack Diamond and Pearl. Reg comes out of its Poké Ball once again to fight it a second time. As their battle continues, several other legendary Pokémon get into the fight. During the battle, Reg, along with several others, are put to sleep by Darkrai's Dark Void; however, Shaymin manages to save them by turning their Abilities to Insomnia with Worry Seed, saving them from an attack from Giratina. The battle with Giratina ends when it attempts to escape, leaving only the Pokémon that can fly to be able to chase it back into the real world. After Charon and Giratina are defeated, Reg, along with the other legendary Pokémon, emerges from the Distortion World with Paka and Uji in its hands. It then decides it will continue to stay on Diamond's team.
Personality and characteristics
Little is known about Reg's personality, as it displays no facial expression and only appears during times of grave crisis that it deems as threatening towards the entire Sinnoh region. In these cases, Reg is capable of emerging from its Poké Ball by itself in order to intervene, without the command of its Trainer. In its few appearances, Dr. Footstep analyzed its footprints which showed Reg's loyalty to Sinnoh and willingness to protect the peace, as well as deep lamentation for Diamond when he was supposedly killed by Giratina's Shadow Force.
As a legendary Pokémon, Reg has demonstrated immense strength which allows it to take on other legendary Pokémon and stop them in their tracks, although it can get worn out over prolonged battling.
| Using Crush Grip
| A † shows that the move was used recently, unless all moves fit this case or there are fewer than five known moves.
For more information on this Pokémon's species, see Regigigas.