Grimm is great, because he's not trying to be cool. Not only is he a lame nerd who is out of place in a story about heroic pilots, he's written so that he knows he's a lame nerd who is out of place in a story about heroic pilots. Everyone else is written for ＧＲＡＶＩＴＡＳ or else specifically to be likable, which makes Grimm almost moe in comparison. Grimm is not only not your worst wingman in AC5, he's probably your best.
PJ is better though.