Kingdom of Lux!

After leaving the home island of Usopp with their new ship the Going Merry, the new crew had rescued a young woman, called Michell D Iris, from the Ocean and join asks her to join the crew because he thought she was cool, however, one member of the crew notices that something isn't right about her but for some reason doesn't tell the rest of the crew about it. What is it that Iris is truely after?