Super Smash Brothers Ultimate - Before you start reading too deeply into a one-word codename...

I'd put a lot more stock in this if "Jack" didn't make a shit ton of sense as Joker's codename.

They have a point. As much as I'd love "Brave" to end up meaning something, I have a hunch that Nintendo/Sakurai/the dev team is trying to keep any future DLC characters from leaking out. For all we know, the character could have no relation to word "Brave" at all and they're just fucking with us.