My take on how(and why) Zoro accepts Robin as a nakama.
Robin is used to the way the swordsman will grasp his sword when she comes too close to him. She has been through far worse for her feelings to be actually hurt. If anything, she is mildly surprised, that the man who shows such implicit trust to his Captain refuses to extend that same trust towards the Captain's decision of making her the newest nakama.
Again, Robin is used to the way the swordsman will never sleep in her presence, silent but tense, and suspicious. But again, she has been through far worse for her feelings to be actually hurt. So like usual, she tries to hand him the blanket when he is spending his time leaning against the rail and trying to sleep.
When Zoro accepts the blanket, she is truly surprised.
"Swordsman-san?" Robin inquires.
"Thanks," Zoro rolls the blanket up to use it as a pillow. "And I'm not just talking about the blanket."
The two older members of the Straw Hat Crew stand in silence as they remember the previous day. They were running low on provisions and when the sea monster had appeared, everybody was cheering. Luffy was the first to propel himself into the air to take the creature on, and so everybody, slightly feeling sorry for the hunter that was soon going to be hunted, just watched.
When the sea monster spitted out a black substance that blinded and thus paralyzed the rubber boy, everybody reacted instantly in their own special way; excessive swearing and panicking, shooting, kicking…
Robin herself had tried to use her Hana Hana no Mi to catch her Captain but the substance proved to be slippery as well. The only thing her hundred arms had caught, was air.
Luffy!
When her powers proved to be of no use, she did the only thing she could think of; run to catch the captain herself.
"You caught him Swordsman-san," Robin points out. Zoro had left the sea creature to Sanji and Usopp, shoved the rest out of his way and jumped off the ship to grab a fist full of red vest so the two would fall together. "I wasn't able to save him."
The swordsman looks up and meets the blue eyes, not a feat many dare to do. His gaze is serious and careful like always.
But there is no suspicion.
"You care enough to try."
The sincere answer takes Robin by surprise. She blinks once before breaking into a small smile.
Zoro had never doubted Luffy's decision; the very fact that he had tolerated Robin being on the ship-even when he wanted otherwise-was proof. However, he was Luffy's First Mate. He knew exactly how trusting Luffy could be and how it could be used to the rubber boy's disadvantage. The swordsman wasn't willing to risk his Captain's safety-he had to see for himself.
He leans back into the pillow he has made as he closes his eyes with an indifference that Robin finds strangely flattering. The First Mate closes his eyes in a clear dismissal but offers, almost as an explanation to his newest nakama.
"That's enough for me."