It is not known to evolve into or from any other Pokémon.
It is often considered to be closely related to Ho-Oh, even sometimes as its polar opposite. Silver represents Lugia as "guardian of the seas", and Gold represents Ho-Oh as "guardian of the skies."
Lugia is the version mascot of Pokémon Silver and its remake, SoulSilver, appearing on the boxart of both.
In Pokémon XD, Shadow Lugia, codenamed XD001, is a main part of the storyline, and Cipher's ultimate Shadow Pokémon. In this form, it is the mascot of Pokémon XD, making it the mascot with the largest number of games, with three games.
Lugia are very large creatures which bear a resemblance to something between a dragon, a plesiosaur, and a bird. They are primarily pale silver-white, but have blue undersides with slightly varying tones. They have a somewhat beak-like, ridged mouths, although they have teeth on their lower jaw. Their head has a point to the back, and Lugia’s eyes have pointed blue or black spikes on them. Lugia have long slender necks and their bodies are smooth and streamlined, with the exception of dark blue or black spike-like protrusions which run down the length of their back, and two smaller ones on the ends of their tails. It has large wings that resemble hands.
Lugia is the leader of the legendary birds. When the they fight over territory, Lugia is the only Pokémon that can quell their ceaseless fighting, as observed in the second Pokémon movie. Much like the birds, it possesses the ability to control the weather; most notably it can calm and give rise to storms. It is said that a light flutter of their wings is capable of causing winds powerful enough to tear down cliffs. If it were to flap its wings it could hypothetically spawn storms lasting as long as 40 days. It is highly intelligent, and because of the devastating power it could accidentally inflict, it isolates itself deep underwater and tends to sleep in solitude. It can shoot incredibly powerful beams from its mouth when in danger and is constantly protected from harm inside a deflective sphere of psychic energy. Lugia is also shown to be able to use telepathy, as seen in The Power of One, while communicating with Ash.
Lugia is the only known Pokémon that learns Aeroblast.
Not very much is known about the behavior of Lugia, as they are fantastically rare and elusive Pokémon. Lugia have been on occassion observed with their offspring, suggesting that unlike other legendary Pokémon, they have a breeding population. However, this is only shown in the anime, and whether or not they can breed in the games is still unknown. They are generally known to be peaceful, protective, calm, modest and benevolent; it is believed the reason they tend to isolate themselves from civilization is to protect humans and other Pokémon from their great powers. However, like all creatures, they can become enraged if they feel threatened, particularly if they are protecting their young. Because of this, contact with Lugia is best done carefully, most attempts at capturing one of these magnificent creatures has ended poorly, and is ill advised as their role in controlling world climate is still not well understood.
Lugia usually live very deep on the ocean floor, not because they are natural sea-dwellers, but because they fear that they can accidentally cause untold devastation. Lugia are typically spotted near the Whirl Islands of Johto and, when disturbed, flying high in the sky. Lugia also found in Metreon City, as listed in Super Mario: The Mystery of HeartGold and SoulSilver.