Regigigas is a large, white, golem-like Pokémon with seven black circular "eyes" arranged in a specific pattern. The pattern is Regigigas's way of showing its anger; its eyes glow red when it is provoked. It has six spots that are apart from its eyes, which appear to be gemstones. These gemstones seem to represent the original three Legendary titans, with red gems representing Regirock, blue gems indicating Regice, and silver gems representing Registeel. Regigigas has long arms, with three fairly human-like white fingers, and short legs that end in large mossy bushes instead of feet. Regigigas has large yellow bands on its shoulders and wrists, with a sloping section on its chest that appears to be its head and is also yellow. Its body is covered in black stripes, and it has moss growing in its back and feet.
Regigigas is a skilled craftsman. It created golems out of inanimate objects and elemental energies, bringing them to life. Regigigas is also capable of controlling these Legendary titans, even if they already belong to a different Trainer. It can also survive extreme conditions as it is able to work with the boiling temperatures of magma (1300-2400 °F) as well as frigid ice (-328 °F). When Regigigas is disturbed from its slumber, it goes on a rampage and shoots powerful beams of energy. When it is befriended, however, it is calm and gentle, as seen in Pillars of Friendship!. It is able to crush targets by using its signature move, Crush Grip. According to Sinnoh legend, Regigigas's strength enables it to move continents.
Regigigas debuted in Giratina and the Sky Warrior. It lives in a temple in the mountains of Ten'i Village and had been asleep for centuries, subsequently having moss growing on its feet and shoulders. However, when Zero entered the Reverse World and began shattering pillars of ice within it, the glacier in front of Regigigas' temple suffered catastrophic explosions and began moving. This disturbed its slumber, rousing it into action. When Dawn's Buneary and Swinub failed to stop the glacier with the help of a large group of wild Pokémon, Regigigas appeared and commanded a herd of dozens of Mamoswine to hold the glacier back. Once the crisis was resolved, Regigigas returned to its temple. It was last seen using Hyper Beam on Team Rocket during the ending credits.
Regigigas made its main series debut in Pillars of Friendship! at the Snowpoint Temple. J wanted to steal it, and a result, it was rudely awakened. It went on a rampage afterwards, using Hyper Beam on anything. It even used Confuse Ray to control Brandon's set of Legendary titans and force them to help it. At the end of the episode, it healed Brandon and his titans using Hidden Power to restore them from their petrification. It was then taken back to the temple to return to its slumber.
Regigigas's silhouette was first seen inside the Snowpoint Temple in Bogging Down Quagsire. When Platinum, Maylene, and Candice were about to freeze in a blizzard following their defeat at the hands of Jupiter, it was the one who carried them to safety. Whilst in her trance, Platinum unconsciously rolled over an empty Poké Ball to the creature, and it ended up in her bag before choosing to join Diamond in an effort to stop Dialga and Palkia's clash. It was later named "Reg."
In Black 2 and White 2, however, the remix of the Legendary titan battle theme is used for Regigigas.
In Platinum, by hacking to obtain Cherish Balls, it is possible to trick the game into opening Rock Peak, Iceberg, and Iron Ruins: by catching the in-game Regigigas in a Cherish Ball, the game thinks the Regigigas was obtained at an event.
In Generation IV, Regigigas is the only Pokémon required to acquire the diploma that cannot be obtained without the use of Pal Park or an event, even with all Generation IV titles. This limitation occurs because Regigigas cannot be obtained without the use of the other Legendary titans, which in turn cannot be obtained without either transferring them from Generation III or obtaining them in Platinum with an event Regigigas.
Regigigas and Eternatus have very similar category names in Japanese, with the only difference being in how they're written: Regigigas's category name is in hiragana (きょだいポケモン), while Eternatus's category name is in katakana (キョダイポケモン). In either case, the category name is pronounced the same: Kyodai Pokémon.
Regigigas seems to be based on the golems of Hebrew legend. The legends cast them as servants of higher powers and are said to have writing on their heads. When the writing is removed, the creature would be weakened, or even killed in some legends. It may also be based on the Titans of Greek mythology. It could also draw some inspiration from the Daidarabotchi, a gigantic yōkai, due to its ability to easily manipulate mountains, while also notably sharing similarities to Ōmitsunu, a demigod who pulled land via ropes to expand his kingdom. Its form also resembles that of a Blemmyes.
Regigigas also seems to have parallels to the Legendary Golem of Prague, whose immense power also intimidated people. This ultimately resulted in it being overrun and sealed away in an attic. Conversely, Regigigas was sealed in a basement. Regigigas being sealed could also be a reference to the aftermath of the titanomachy, which resulted in the titans being imprisoned away in the underworld.
Regigigas may be a combination of regis (Latin for royal) and gigas (Latin for giant).