From Halopedia, the Halo wiki
Private Kappus was a United Nations Space Command Marine technical specialist. He was assigned to Fireteam Charlie, the unit that also housed Privates Manuel Mendoza, Bisenti, Riley, Private First Class Jenkins, and Staff Sergeant Avery Johnson.
As a tech specialist, one of Kappus' tasks was opening locked doors with a spoofer. During the UNSC raid on the supposed Covenant weapons cache on Installation 04, he was ordered by Captain Jacob Keyes to open the door that just so happened to be the only thing holding back the Flood. Unwillingly and unknowingly, Kappus unleashed the Flood on Installation 04, dooming him and the rest of his squad, save for Johnson. When Riley was attacked by a Pod infector, Kappus successfully managed to pull it off, but others soon found Riley. Kappus was one of the first to be transformed into a human combat form.
The Flood-infected Private Kappus was probably killed by the Master Chief during the latter's escape from the installation, by the UNSC Marine or Covenant forces, or when the Chief destroyed Halo.
His role in the game may be mistaken for that of Mendoza's due to the same character model seen in-game.
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