Calyrex is a bipedal Pokémon with legs approximately twice as long as its torso. Four prongs grow from its head to support a green bud resembling a type of crown, much larger than its head. A ring of green spheres, marked with an inlaid series of blue and white triangles, adorns its neck. When it is in Ice Rider/Shadow Rider form using the Reins of Unity, the green fur on Calyrex's arms and back extends to resemble a white and green cape. The crown-like bud on Calyrex's head glows bright green when it is using its powers.
Calyrex is said to have ruled Galar in ancient times as a merciful king. Despite its delicate and slight appearance, it can move with grace and dignity. It is known to be highly intelligent; Calyrex is one of the few Pokémon able to fluently understand and telepathically communicate the human language. It also has the power to see the past, present, and future of any kind when at full power. One legend said Calyrex used its foresight ability to save creatures of a forest from a meteor one time. One fairy tale even mentions Calyrex stealing the body of a human that causes mischief. Calyrex has the ability to take control of a human's body in order to communicate and speak telepathically when at full power.
It has been referred to as the "King of Bountiful Harvests" due to its role in bringing lush vegetation and harvests yearly. Calyrex's right hand can cause the land to be covered in verdant grass and blooms, while its left hand allows it to make fields of fruit ripen and become heavy. Calyrex can create either Iceroot or Shaderoot Carrots, depending on the field where the Carrot Seeds are planted, with a special dance. Calyrex can also create the Radiant Petal needed for the Reins of Unity. It can provide healings or blessings such as mending the heart. While it shows no mercy to those that stand in its way, it will heal the wounds of a foe afterwards. Calyrex is powerful enough to move an entire forest and its Pokémon to a new location.
Calyrex has the power to tame and combine with Glastrier or Spectrier when using the Reins of Unity. This allows Calyrex to be at its full power and potential. Calyrex also can heighten Glastrier's or Spectrier's power with its psychic energy. This in turn allowed Calyrex's psychic powers to manifest up to a 30-mile (50 km) radius. When riding either Glastrier or Spectrier, it ran all over Galar to bring green within the region's wastelands. Calyrex gets its powers from the faith of the people who worship it, and slowly loses its power if people start forgetting it and should Glastrier or Spectrier abandon it.
By combining with Glastrier, it can can emit a cold temperature up to −508 °F (−300 °C). Glastrier's endurance also increases to the point it can run up to seven days and nights. This allows Ice Rider Calyrex to smash and freeze anything in its path no matter the difficulty of the road. Ice Rider Calyrex can use its psychic power to transform Glastrier's intense cold into gigantic icicles. It can use the icicles as other arrows to fire or lances for charging. The icicles are powerful enough to blow holes in rocky mountains and allows the duo pass through it completely.
When fused with Spectrier, it becomes swift to the point it can run a thousand miles within a single day. The strength of Spectrier's spectral power also increases. Shadow Rider Calyrex can infuse Spectrier with its psychic powers in order to create countless small will-o'-the-wisps. It uses the wisps to rain down on others like meteors before it steals the opponent's life-force with burning heat. It can also transform the steed's spectral aura into a black mass. This mass can absorb the life-force and souls of any being's energy like a black hole.
In its alternate forms, Calyrex is the only known Pokémon capable of using the moves Glacial Lance and Astral Barrage.
- Shadow Rider Calyrex has the highest base Speed stat of all Ghost-type Pokémon.
- Ice Rider Calyrex is the heaviest Ice-type Pokémon.
- Ice Rider Calyrex achieves the lowest temperature of all Pokémon, −508 °F (−300 °C), which would be a negative temperature that is impossible in the real world since it is below the absolute zero, −459.67 °F (−273.15 °C).
- Unlike other Pokémon, when Calyrex Dynamaxes, it gives off a blue aura instead of the usual crimson one. Calyrex is the only Pokémon to do this.
- Calyrex is the only Pokémon capable of fusion with another Pokémon that can be captured in its fused state. Previous Pokémon, such as Kyurem and Necrozma, could only be captured when separated into their individual components.
- Calyrex is the only Pokémon that can change between alternate forms at will to have the list of moves it can learn from TMs, TRs, and move tutors change between them.
- When Calyrex unfuses, it will forget not only its signature move, but all moves that are unavailable to its base form.
- Calyrex's base typing makes it weak to the types of both its mounts.
Calyrex may be based on an arctic hare and a crown. The large bulbs on its head and around its neck resemble berries or flowers. It also bears some resemblance to fictional taxidermy creations, such as the American jackalope or the German wolpertinger. Its beads around its neck each bear the Hōjō clan symbol. Conceptually, it may be based on pre-Victorian depictions of fairies and elves, or any one of a number of Horned deities from various European pagan religions, such as Cernunnos or Herne the Hunter. It may also reference the idea of the king in checkers or chess. Furthermore, its silhouette is reminiscent of a thistle, the floral emblem of Scotland, which is the real-world location the Crown Tundra is based on. Its fusion forms appear to be a reference to cavalry, or the folklore of the Wild Hunt and is visually similar to the Nuckelavee.
Calyrex may be a combination of calyx (Latin for bud) and rex (Latin for king) or carex.
Budrex may be a combination of bud and rex (Latin for king).