Solgaleo is a large, white Pokémon resembling a lion. It has a blunt muzzle with a blue-gray nose and pale blue eyes. The top half of its face is a deep blue area that shows a constantly changing starscape. Four yellow spikes frame Solgaleo's head like a crown and two more pairs of spikes adorn either side of its lower jaw. Each spike separates large tufts of Solgaleo's mane. Solid ridges—red nearest Solgaleo's head and yellow farther away—poke out of each tuft along the middle, except on the tufts on its cheeks and chin, with the topmost part wrapping around the tip of the tuft. Overall, its mane creates the impression of a stylized sun. Around each leg is a dark orange band, and there is a black spot on each foot. It has three black toes on each foot with long yellow claws. At the tip of its tail is a large black spot with a thin yellow rim.
When Solgaleo enters its Radiant Sun phase, its third eye appears on its forehead—a light blue symbol similar to a compass rose—and the starscape turns to streaks of light. The fur on its body glows a pale yellow and all other parts of its body except its nose turn bright yellow.
Known as the "beast that devours the sun," Solgaleo has long been honored as an emissary of the sun. It said to live in another world and returns there when its third eye activates. Its body is capable of holding a vast amount of energy and glows when active. The intense light it radiates can make nighttime look like midday. Solgaleo can create Ultra Wormholes to travel to and from Ultra Space. Along with its counterpart Lunala, it can create Cosmog, which it is said to be the male evolution of. It can also influence a Rockruff's evolution into Lycanroc's Midday form with solar energy. 
In Full Moon and Many Arms!, Solgaleo was involved in rescuing Lunala and Necrozma from Team Rocket's captivity. Later on, after Necrozma separated from Lunala, it was taken over by Necrozma, becoming Dusk Mane Necrozma. It then opened up an Ultra Wormhole to Poipole's world. Ash tried to follow Solgaleo (now fused with Necrozma), but failed because the Ultra Wormhole closed.
In The Prism of Light and Darkness!, after being pursued by Ash, his classmates, and Lunala, to Poipole's world, Solgaleo was successfully separated from Necrozma after the Ultra Guardians, Naganadel and its Poipole subjects, and all the people and Pokemon of Alola sent their light to Necrozma. However, due to being fused with Necrozma for a long period of time, Solgaleo was completely drained of energy. When Ash and Mallow gave Solgaleo a Sitrus Berry, Solgaleo regained its strength once more. Ash requested its power, and it and Ash performed the Z-Move Searing Sunraze Smash alongside Lunala and Gladion, who were performing the Z-Move Menacing Moonraze Maelstrom.
Solgaleo's Japanese name and its romanization were revealed in a tweet of their trademark on March 22, 2016.
Solgaleo's physical and special stats are the reverse of Lunala's.
Solgaleo's verbal cry in Pokémon Sun, "La-liona", may be based on the words lā (Hawaiian for sun) and liona (Hawaiian for lion).
Solgaleo appears to be based on a lion with a sun motif, a fairly common motif in many Western and Middle Eastern cultures. In particular, it might reference the alchemical lion, being described as "devouring the sun", however typically a green lion and a metaphor for aqua regia, as well as connected to metals via its Steel type (matter). In astrology, the Sun is also the ruler of the sign Leo.
As the emissary rather than a personification of the sun, as well as being able to pass through different dimensions, it might lend some inspiration from the eye of Ra, a goddess usually described as a lion that is the sun's protector in Egyptian mythology as it travels through the underworld. Its regular and Shiny colors may be inspired by phases in stellar evolution: regular Solgaleo may represent the Sun as a white main-sequence star, while Shiny Solgaleo may represent the red giant that the Sun will eventually become as the hydrogen inside its core is exhausted.
Solgaleo may be a combination of sol (Latin for sun) and leo (Latin for lion). It may also incorporate Galileo, an Italian astronomer who presented evidence that the Earth rotated around the Sun and advocated Copernican heliocentrism.