From Halopedia, the Halo wiki
John-117, better known as the Master Chief, is the main character of the Halo franchise, and the protagonist of Halo 3. A SPARTAN-II, he was kidnapped at the age of six by the UNSC's Office of Naval Intelligence and trained from a young age to be the "ultimate soldier", before being given biological augmentations and powerful MJOLNIR armour. The Master Chief was responsible for destroying the ancient superweapon of Installation 04, before eventually fighting for Earth and leading the assassination of the Prophet of Regret on Delta Halo. At the culmination of the conflict, John smuggled himself aboard the Prophet of Truth's dreadnaught to return to Earth and finish the fight.
Thel 'Vadam, more commonly known as the Arbiter, is a key supporting character of Halo 3 and a key ally of the Master Chief. This disgraced Sangheili warrior was the former commander of the Covenant fleet encountered in the first Halo game, before being branded a heretic by the Covenant for his failures. The Arbiter now seeks to lead his people against the Prophet of Truth and his Brute allies in revenge for the Prophets' lies, and forge an uncertain alliance with humanity.
The Prophet of Truth is the final survivor of the three Hierarchs who lead the Covenant. Having orchestrated the Changing of the Guard and the ensuing Covenant civil war, the Prophet is hell-bent on activating the Halo rings to ascend into godhood. The Prophet departed High Charity at the end of Halo 2 with the Master Chief in tow, and is now laying siege to the Earth with his armies of ever-loyal Brutes.
Commander Miranda Keyes is a UNSC Naval officer and the daughter of the late UNSC Pillar of Autumn Captain Jacob Keyes. Miranda was the commander of the UNSC forces deployed to Delta Halo, before being captured by the Covenant and eventually pivotal in forging the alliance with the Arbiter and his Elites. Keyes now leads the human resistance forces on Earth from a hidden facility deep within the jungles of Kenya.
Cortana is a human artificial intelligence, who served as a companion and friend to the Master Chief throughout the first two Halo games. She was created from a clone of the brain of renowned scientist Dr. Catherine Halsey, once the director of the SPARTAN-II program. During the conflict on Delta Halo, Cortana was left behind on the Covenant capital-station High Charity to ensure the destruction of the parasite if necessary, while John-117 returned to Earth alone. Cortana's whereabouts are now unknown, though her accrued knowledge and experience may hold the key to ending the Covenant and the Flood permanently.
Sergeant Avery Johnson is a UNSC Marine Corps veteran and long time friend of John-117. The Sergeant was involved in several major anti-Insurrection missions prior to the war with the Covenant, and was one of the first humans to encounter the Covenant on the planet Harvest in 2525. Since first contact, Johnson has fought alongside the Master Chief and his fellow SPARTAN-IIs on dozens of campaigns for nearly twenty-eight years, and now joins John-117 in defending the most important planet of all - Earth.
343 Guilty Spark is the ancient AI custodian of Installation 04, built by the Forerunners. Although the years have taken their toll on him, he remains dedicated to protecting the array and ensuring the prevention of the spread of the Flood. While he opposed John-117's efforts in Halo: Combat Evolved, he now stands as an ally of the UNSC and the Elites as long as they are dedicated to stopping the Prophet of Truth and the Gravemind.
The Gravemind is the centralised hive-mind intelligence that leads the armies of the Flood parasite. Imprisoned on Delta Halo for millennia, the Gravemind was able to escape following the outbreak of conflict on the ring, managing to find its way onto High Charity. The intelligence has since converted the Covenant holy city into a mobile hive capable of sundering entire planets, and is now preparing to lead a massive attack on Earth.