Question

Is HBr a nucleophile?

Answer 1

Pretty much never.

A nucleophile MUST be a Lewis base, and there is a very poor chance that `"HBr"` will donate electrons BEFORE it donates its proton; its pKa is about `-9`, i.e. it's a pretty strong acid.

It is much, much more likely to give up a proton by accepting electrons. That just shows that it is a Lewis acid, NOT a Lewis base, and therefore it is an electrophile.