Kappus
From Halopedia, the Halo wiki
Private Kappus was a specialist in the squad, that also housed Private Manuel Mendoza, Private Bicenti, Private Riley and Jenkins, not to mention Sergeant Johnson.
He was a specialist in Covenant and Forerunner technology, particularly in opening locked doors. He was ordered by Captain Keyes to open one door in particular, which he hesitated to open. Unwillingly and unknowingly, Kappus unleashed the Flood on Installation 04, dooming him and the rest of his squad, save for Johnson. His role in the game is often mistaken for that of Mendoza's.