Before she officially join the Strawhat Pirates (before Enies Lobby arc), she normally use titles, such as "Miss Navigator", "Mr Long Nose" etc, on the crew members. She knows that she is bound to leave them, hence creating the distance between them. After seeing how the crews went all the way to save her in Enies Lobby arc, she begin to call them by their names.

I am emotionally touched, when she expresses her concern for Ussop and Chopper by calling their names with it, in episode 351 (Thriller Bark arc).


The 7th member, Nico Robin from One Piece

Luffy

Although occasionally, she will address Luffy by "Luffy" directly, I feel this is the first instance where the relationship is strong and firm. There is finally no hidden agenda. After all, this is the scene where she thank everyone for coming to save her sincerely.

Franky

In Thriller Bark arc, episodes 347 and 353 (a flashback) respectively, she called Franky directly. In the scene on the left, she cautioned Franky about the impeding danger behind them. On the right, she was wondering why Franky asked Brook such a question (spoiler?).

Ussop & Chopper


Yep. This is the scene which I mentioned above. During the end of episode 351, Ussop and Chopper were flanked and attacked by a group of zombies until Franky and Robin arrived to save them in the nick of time. I was really touched, hence I decided to start this post. =)

Nami

Before she address Nami by her name directly, she already reference her by her name when talking to other crews when she is not around (on the left). In episode 355, after saving Nami from Oz's stretched arm attack, Robin expressed her concern immediately (on the right).

The Strawhat crews are now in the Sabaody Archipelago arc at the moment =)