From the Super Mario Wiki
A White Magikoopa is a Magikoopa that appears in Paper Mario and Paper Mario: The Thousand-Year Door. Their name is abbreviated as W Magikoopa in Paper Mario and W. Magikoopa in the sequel. Like most Magikoopas, White Magikoopas can shoot magical blasts at enemies, but their specialty is healing either themselves or their allies in battle. They will sometimes ride a broom in battle, which will allow them to hover above the ground and cause hammer attacks to miss. Like all Magikoopas, they will duplicate themselves or run away if alone in battle.
In Mario & Luigi: Dream Team, Kamek creates a white-robed clone of himself called Kamek (White) to assist him in his third battle against the Mario Bros. in Dreamy Neo Bowser Castle. This clone heals Kamek, his other clones and itself just like White Magikoopas.
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