Princess Eclair was the damsel-in-distress that Luigi rescued from the clutches of Chestnut King, while adventuring in the Waffle Kingdom. These adventures take place in Luigi's versions of his story and the book series titled "Super Luigi". These books appear in Paper Mario: The Thousand-Year Door.
In contrast to Luigi's initial belief, Princess Eclair was the king's lover, who had been turned into a monster by the sinister Crepe. This story is told by Luigi when Mario talks to him throughout the events of Paper Mario: The Thousand-Year Door. During Luigi's adventure, the green-clad plumber fell in love with Princess Eclair. However, after he discovered that the Chestnut King was Princess Eclair's fiancé, Luigi lost the hope of ever forming a romantic relationship with the princess.