After ‘Star Wars: The Force Awakens‘ came out, emotions were felt. They were felt hard.
His name is FN-2199. But his friends call him “Nines.” As detailed in Greg Rucka’s excellent book ‘Before the Awakening’, Nines trained and served on a squad with Finn (then FN-2187) in the First Order. He gets an Z6 Baton because he’s part of an elite riot squad who enforces control and squashes uprisings.That explains why he seems just a little extra angry upon seeing Finn during the attack on Maz’s Castle. You can see FN-2199 in the image below by Phil Noto from ‘Before the Awakening’: