A secret, playable character in Galaxy has been discovered and all the spoiler-filled details have been made known.
The official Super Mario Galaxy strategy guide has revealed that upon completion of the plumber's forthcoming outer space adventure, players will be able to take control of a secret character. A scan, containing all the details of the spoiler, is available below.
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The secret character's only playable, as the scan above explains, once all 120 stars have been acquired, once all purple coin missions have been completed and once Bowser has been defeated for a second time.
Source: Aussie Nintendo