Miss All Sunday
Robin first appears as "Miss All Sunday" ("Miss Sunday" in the 4Kids English anime), agent and vice-president of the Baroque Works organization, at the end of the Whiskey Peak arc. She was responsible for blowing up Igaram's decoy ship, though Igaram survived, which apparently was her intention. Princess Vivi revealed that it was Miss All Sunday who allowed her to discover the identity of Baroque Works' president, Crocodile. Nico Robin the second strongest person in this group as well as partners Crocodile. But he kept his nature as "the devil". Until Nico Robin met Straw Hat Pirate Group on cactus island Whiskey Peak. Nico Robin offers them so they stay away from the past Alabasta Nanimonai island, while providing an eternal pose (sound tool). But Luffy destroy Eternal Pose, saying if it was not Nico Robin who has the right to determine their route. After that Nico Robin go. At that time, Luffy and Nami Nico Robin regarded as a good person, and this reference if Nico Robin is a member of straw hat next.
After that Nico Robin reappeared when escorting Crocodile Straw Hat Pirate Group cooped up in Rainbase, Arabasta. Nico Robin defeated Pell, soldiers Arabasta easily then kidnap Vivi. Nico Robin here looks very similar to and harmonious with the Crocodile.
In the Baroque Works, Nico Robin is the same as in other organizations, not considering the existence and treason. Nico Robin Luffy help when going to fall into the pit of sand that made the Crocodile, Nico Robin also tries to lie to him when explaining the existence of an ancient weapon in Poneglyph plutons, but Nico Robin after he defeated Crocodile know if Nico Robin lied, The last time Nico Robin Luffy's rescue again with give the antitoxin for toxin from Crocodile.
And fter the destruction of BW, quietly slipped Nico Robin (Nico Robin Luffy saved from death) on board the Going Merry. Then Nico Robin tells Luffy if Nico Robin will join the group. Luffy just answered "Yes" while the other friends who refuse to upset because they still suspect because he was a member of BW, except for Sanji as love with a beautiful woman.
Source Nico Robin