Kusovai is an Elite character first introduced in the Halo Graphic Novel. His naming convention doesn't follow the normal elite format of firstname 'lastname suggesting that Kusovai is a rank or a nickname.
In context
You see Kusovai when Half-Jaw boards the Infinite Succor and realizes that something is amiss. When they head into a hunting preserve there are Flood infested animals that begin attacking the group. The group fights its way through to the ship's bridge, where they find the controls have been disabled.
Kusovai is eventually absorbed by the Flood and fights Half-Jaw. The Flood are almost able to Slipspace jump out of the system but not before Half-Jaw changes the course to slipspace into the sun. It was here that Half-Jaw while fighting an infected elite an Energy Sword cuts off his left mandible. Half-Jaw managed to escape two minutes before the Slipspace Jump occured. Half-Jaw is believed to be the only survivor of the Infinite Succor mission.