Halo 2 introduced the Arbiter as a playable character.
Released in 2004, Halo 2 expanded the series by allowing players to experience major parts of the story from the perspective of a Covenant Elite. The character begins as the disgraced former Supreme Commander of the Covenant fleet and is assigned the title of Arbiter by the Covenant’s religious leadership.
The game alternates between the Arbiter’s missions and those of Master Chief. This dual perspective reveals that the Covenant’s leaders have manipulated their military and religious institutions, while also showing the growing alliance between the Arbiter and humanity.
A common mix-up is to identify the Arbiter as a separate species or title introduced in Halo 3. The character first became playable in Halo 2, while Halo 3 continued his role and made the human-Covenant alliance central to the conclusion of the original trilogy.