After Robin Joins the Crew

Nico Robin was interested in Monkey D. Luffy. He had the Will of D. like the giant she met long ago and who helped her escape the buster call at Ohara, Jaguar D. Saul, and Luffy apparently as his first travels into the Grand Line became an enemy of a crimnal organization headed up secretly by one of the 7 Warlords, Sir Crocodile and eventually did defeat him.

After Robin gave him the antidote to Crocodile's poison and he helped her escape from the falling debris when she wanted to die, she snuck on board the Going Merry and after the Straw Hat Pirates escaped, she revealed herself and asked to join the crew, which Luffy accepted.

But just because all this was true, Robin knew that she couldn't just trust Luffy and the other Straw Hats that easily. The World Government of course would always be after her and although they seemed like good people, what would happen when they realized the true power of the WG with buster calls, admirals, and so so much more up the WG's sleeve?

If she was going to be on this crew for a while, then she needed to influence Luffy in a way so he would keep her out of the WG's radar for as long as possible and so he would look to her for guidance above even the rest of the crew. After some time spending on the Going Merry, she had an idea and decided she would feminize Luffy.