All of the looks for the Arbiter, coming to Killer Instinct's Season Three next month, have been revealed by 343 Industries in Halo Waypoint's latest update. The fighter's weapons and move sets are also discussed in detail.
As previously indicated, this character is not voiced by Keith David, making it an Arbiter and not the Arbiter. Halo's canon allows for this as Arbiter is a rank (of the Sangheili) not necessarily a specific being, though "The Arbiter" is the more commonly known name of the one voiced by David since his appearance in Halo 2.
So, according to Frank O'Connor, the franchise development director for Halo, when 343 chose to bring an Arbiter to Killer Instinct, they went with a "Hall of the Arbiters" concept, and that means a lot of looks for this fighter. They are: Kaidon, Arbiter, Combat, Storm and Ranger.
You can see them in the gallery below. 343 also discussed Arbiter's moveset, which will use the Type-51 Carbine, Plasma Grenade and Energy Sword as weapons. His stage will be the Arena of Judgment; concept art for that is below.
Season Three will launch sometime in the coming month on Xbox One. New characters announced for its roster, in addition to the Arbiter, are Kim Wu, Tusk and Gargos from Killer Instinct 2, and Rash, a special guest star from Battletoads who was briefly playable in Season Two, but only as a test character.