Cresselia is a Pokémon that has a blue, swan-like body with a yellow underside. It has pink eyes, a dark pink, round tuft on its head, and a pointed, beak-like snout. There are yellow, crescent-shaped ornaments on the sides of its head that merge together in the back. It has a pointed, tail-like protrusion on its rear. Cresselia has glowing, pink, ring-like wings on its sides and back. The wings on its sides are solid dark pink, while the wing on its back is larger and consists of three shades of pink. These wings, especially its back wing, release shining particles and undulate beautifully around the quarter moon. It has paw-like protrusions resting on its chest.
Due to its appearance, Cresselia is said to resemble a crescent moon and the woman who made the Milky Way. Cresselia has the power to dispel nightmares, and holding one of its feathers is said to bring happy dreams. It is also great at recovery and healing techniques. Cresselia can unleash spores in the air to cause confusion on opponents while forming a shield to defend itself at the same time. Its relationship to its counterpart, Darkrai, can vary between individuals and canons, with some being antagonistic, as seen in Sleepless In Pre-Battle! and Pokémon Mystery Dungeon: Explorers of Time, Darkness, and Sky, or cooperative, as seen in Nightfall? Nightmares! and A Midsummer Night's Light!. It tends to reside on a island or mountain that has a Darkrai nearby in order to keep an eye on it.
A Cresselia first appeared in Nightfall? Nightmares!, where she was targeted by Team Rocket's Matori Matrix. At the end of the episode, she was found by Chloe and Dawn. In A Midsummer Night's Light!, Dawn helped to treat Cresselia's wounds. When Matori Matrix attempted to capture Cresselia, Darkrai defended it, but was badly hurt in the process. Cresselia used Lunar Dance to heal Darkrai, who defeated Team Rocket before clearing up the clouds, allowing Cresselia to heal herself with Moonlight.
Cresselia appears on the Spear Pillar stage as one of three Pokémon who appear in the center (the others being Dialga and Palkia). Cresselia acts completely different than the other two in terms of attacks it uses. It will often use Psycho Cuts around the field, and sometimes shoot one large Psycho Cut as if it were a boomerang, hitting anyone that touches it.
A Lunar Pokémon. Cresselia appears on Fullmoon Island and, once discovered, flies around different areas of Sinnoh. Its signature move, Lunar Dance, causes Cresselia to faint but recovers all the HP and cures the status conditions of the next Pokémon its Trainer sends out. Cresselia excels at recovery techniques and is unusual in that it appears only in female form.
Cresselia reprises its role as a stage hazard on the returning Spear Pillar stage. Spear Pillar now allows for any of the three Legendary Pokémon to appear each time, rather than being set for the whole match; however, Cresselia appears much less frequently than Dialga and Palkia. Unlike in Brawl, the stage now has a green color tint when no Pokémon is active, and Cresselia's appearance does not change the color from this default green; in Brawl, Cresselia used the same blue tint as Dialga to signify its appearance. Cresselia also appears as a Spirit.
In the English Demo of Diamond and Pearl, Eldritch refers to Cresselia as the Quarter Moon Pokémon.
Cresselia's form seems to be based on a swan. Considering its crescent moon motif and the aurora-colored planetary rings around its body, it may originate from the constellation Cygnus. The rings also resemble the hagoromo gowns worn by the Tennin, spiritual beings in Japanese Buddhism, and its colour scheme of pink, yellow, and light blue, is the same as the dress worn by the Chinese goddess of the moon, Chang'e. Cresselia shares characteristics with Sarimanok, a legendary fowl with a profusely decorated head, feathered tail, and colorful wings that are said to be a symbol of good fortune. In Cresselia's Hisui Pokédex entry it states: "[Cresselia] resembles the heavenly maiden who created the Milky Way", this "heavenly maiden" may be a reference to the Estonian goddess of birds Lindu who is said to have created the Milky Way with her tears.
Cresselia may be a combination of crescent (referring to the lunar phase) and Selene (Greek goddess of the Moon).