Arbiter Thel 'Vadam (formerly "Thel 'Vadamee"), often simply referred to as The Arbiter amongst players, was a distinguished Sangheili. At the apogee of his career with the Covenant he served as the Supreme Commander of the Fleet of Particular Justice. However, following the destruction of Installation 04, he was stripped of his rank, and forced to don the ancient armor of Arbiter. He accepted the position of Arbiter to continue his service to the Covenant, even in his disgraced state. The Arbiter is always sent on a suicidal mission to claim great honor in death, so he was not expected to live. However, the Hierarchs (and later the Jiralhanae that the Prophets used to attempt to assassinate the Arbiter) underestimated his skill, and he survived. He, along with the rest of the Sangheili, allied themselves with Humans, after the Gravemind told him of the Halo's true purpose, and was betrayed by his Covenant brethren. He later swore vengeance on The Prophet of Truth.
Simply, Arbiter is god.