That's "Ninja Ninja"... voiced by Samuel L. Jackson.
It's like Afro's interior dialogue, or... other half so-to-speak.
basically, he's there to give all of Afros more 'human' reactions (IE: OH SHIT! OH WHAT THE FUCK IS THAT?! OH GAAAWD DAAAAYM HE'S GOTTA RPG!!), rather than just this super quiet dude who slays shit to smithereens.
Have a nice day, Bluename.