It is revealed in HeartGold & SoulSilver chapter that Reg was initially awakened by Palmer with the help of Brandon's Pokémon so that it could be used to stop a legendary crisis 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 in silhouette after Platinum, Maylene, and Candice end up camping there after their defeat at the hands of Jupiter. It lets loose a growling noise that startles 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.
Later, during the final battle with Cyrus on the Spear Pillar, Reg appears from out of Platinum's bag to save Diamond from entering a black hole. After saving him, Reg chooses Diamond as its Trainer and they go to battle against Dialga and Palkia. Reg manages to grab Dialga and Palkia and pin them down so that the black hole they created while fighting will not be able to widen any further than it already has. Although it was able to stop them briefly, Dialga and Palkia are encouraged by Cyrus to fight even harder soon after. The power they expel is 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 the Platinum chapter, Diamond reveals 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 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.
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.
Later, at the Distortion World, when Charon sends Heatran 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, is put to sleep by Darkrai; 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.
Later, 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 is decided that Reg will continue to stay on Diamonds' 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.