Dr. Alexander was a human scientist stationed at Galileo Base.
Dr. Alexander used the power supplies of the defenses of Galileo Base to run a mini reactor. As such, the base defenses went offline. Not long after, Galileo Base was attacked by Covenant forces.
Fireteam Crimson was sent here to defend the base and kill the attacking forces. After the attackers were dead, Dr. Alexander explained that if the mini-reactor isn't turned on again, it will explode and wipe Galileo Base of the map.[1]