He was a Tech Specialist in Covenant, Forerunner, and UNSC technology. One of his tasks was opening locked doors. He was ordered by CaptainJacob 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 and Bisenti. When Riley was attacked by an Infection Form, 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.
In Halo: CE, his role in the game is often mistaken for that of Mendoza's due to the same character model seen in-game, but that was fixed in Halo: CEA as 343 used a different model for Kappus and Mendoza.