O'Brien (The Babysitter)

"I don't miss! I never miss!"

- O'Brian

O'Brian was a member of the UNSC Marine Corps who participated in the Human-Covenant War. He was an Orbital Drop Shock Trooper assigned to the Marine contingent aboard the Destroyer DD-993

Assassination
He and his fellow Helljumpers were charged with attacking a Covenant force on an unnamed planet, in which O'Brian was to serve as the backup sniper. He felt this was an insult to his abilities and was even more shocked to learn that a Spartan-II was the primary shooter. During the operation, he is attacked by a Brute Chieftain that was wielding a Gravity Hammer, and is almost killed. Cal-141 saves him, but O'Brian winds up hanging over a waterfall as he watches the Spartan battle the creature. Cal saves him again when she pulls him up off the waterfall, he seemed unappreciative as he yelled at her to put him down. The group soon makes its way to where they're taking the shot, as Cal is about to take the shot, the Chieftain from earlier returns and is about to kill O'Brian when Cal saves him again at the last second, taking the blow from the Chieftain's hammer. The ODSTs tackled the Chieftain to the ground and Cortez shoots the Brute in the head. Cal then yells to O'Brain to take the shot, at first he hesitates but takes up the Sniper rifle and takes out the Prophet. He is later seen back on the Destroyer, not saying a word, and thinking deeply about how Cal continually saved his life.

Trivia

 * O'Brian bears a striking resemblance to Spike Spiegal from the anime, Cowboy Bebop.
 * O'Brian is one of the few known Humans to kill a Prophet.