...oh! [This is comprehension dawning on him.] That kind of hero! A superhero!

[Chopper Mask would love this. Usopp, too, maybe. As the grandpa figure of their little pirate family, Brook is surprisingly genre savvy.]

You have to have a costume if you want to be a cool superhero, Sanji-san. [And then back to Kotetsu:] And it helps to safeguard your secret identity, correct?