Sai wasn't the best when it came to reading people. This had a lot to do with his own lack of emotions. He did best when reading those who were nearly as stoic as he was. He simply followed his orders down to the finest detail, that was all. When he lead a mission it was usually with others from ROOT, and they all had their goal as well as he did. Weaknesses weren't tolerated.

But there was always his brother, and Naruto, and others that he'd met outside of ROOT that had certain characteristics about them. Certain ways, and a certain innocence to them. Usopp had this. Mello did not. At least not what he'd seen so far. The way the man acted - he didn't seem frightened by him, even though he'd been pinned to the wall moments before. He wasn't nervous and he was already showing how confident of himself he was. He was ready to do business already. Sai was ready to regard him as he would opposition in a mission, but he shouldn't do that. Not here, where alliances were more important. He couldn't be so quick to judge.

"Usopp here likes to help others," he said, motioning to the pirate with a short gesture. "As his friend and ally, I help him. He is a craftsman. You are alone without allies. Usopp, weren't you looking for others willing to work with us?"

Mello didn't appear entirely trustworthy, no. But he had a hardness that his pirate friend lacked, and that could be useful.