[While Zoro can't specifically see Ace, he can see the ripple in the air, like a mirage, suggesting where a ghost might be. It's a little less tangible than some of them have been, maybe because it's focusing its energy on Luffy.

There's no way to know if anything he does will have an effect, but might as well try. Zoro strides over, pulling his bandanna free and gently pressing it into Luffy's hand as he passes and places himself back to back with his captain.]

I don't know who you are, but you're not who you look like. You want at him, you're gonna have to go through me. Show yourself, you rotten bag of wind.

[probably not anywhere near threatening enough. But he's adamant and will stand his ground.]