The relationship between Roronoa Zoro and Nico Robin of the One Piece series has formed a very intricate, changing dynamic that has held the interest of many fans. In their interaction, mutual respect and gradual building of trust show just how much these two characters alike have come along.
When Robin first joined the Straw Hat Pirates, Zoro was suspicious and wary of her. Even on their first meeting, he told her that he would not take his eyes off her, which proved that he strongly suspected her. It was a very Zoro-ish response since he is hostile toward new members in general. On her part, Robin simply smiled, defining the atmosphere of their relations at the beginning.
Zoro's initial distrust of Robin was not unfounded, given her mysterious background and former allegiance to Crocodile. However, as the series progressed, it changed. How Zoro slowly begins to look upon her as a trustworthy member of their crew becomes quite vivid through various character interactions and cooperative efforts across several arcs.
One of the noted things about Zoro and Robin's relationship is their very similar temperaments. Both characters are often portrayed as more mature and level-headed compared to some of their crewmates, who can be more exuberant. This has led to many quiet moments of understanding between them, especially in highly pressurized situations where these calm demeanors come in very handy.
Through the series, Zoro definitely has exhibited acts of protection on Robin. One case in particular is during the Zou arc, where Carrot suddenly attacked Robin; Zoro was so fast to shield her, which demonstrated his protectiveness toward his crewmate. Such acts, though not uniquely directed at Robin, pointed out the trust and brotherhood that have grown between them.
The development in Zoro and Robin's relationship is most often compared to other iconic pairs within the One Piece world. Most of the comparisons liken their pairing to Silvers Rayleigh and Shakuyaku, focusing on both couples apparently sharing the same role and dynamics. However, these comparisons are only theoretical and not entirely validated in the series.
This can be proved when it comes to their jobs within the crew: Zoro and Robin complement each other very well, with him providing strength and sword fighting and her providing brains and historical knowledge. The complementing nature of their skills was noted in different situations. For example, episode 770 shows a time when they stood out compared to their companions because of their competence.
This development of Zoro's and Robin's bond was rather significant, considering Zoro's character. For a first member to join the Straw Hat Pirates and a master swordsman like Zoro to actually accept her, that really sealed the deal with regard to his growing character and the strength bonding shared among the crew. His initial suspicion giving way to trust and teamwork shows the general theme of found family within the Straw Hat Pirates.
Although some fans have tried to find hidden meanings in certain interactions between Zoro and Robin, it is considered that the series hasn't shown any direct sign of their romantic relationship. Their interaction is mainly based on mutual respect, trust, and their roles within the crew.