The replacement protagonist in Halo 3: ODST is known as the Rookie.
The Rookie is an unnamed Orbital Drop Shock Trooper serving with the squad during the Covenant occupation of New Mombasa. His pod is separated from the others during the disastrous orbital drop, and he wakes several hours later in the city. The game then follows his search for his missing teammates through a nonlinear investigation.
Players control the Rookie during the nighttime exploration sections, while flashback missions place them in control of Buck, Dutch, Romeo, and Mickey. This structure explains what happened to the squad after the drop and gradually reveals why the city became such a dangerous battlefield.
The Rookie is not Master Chief and is not a Spartan. He is an ODST, a highly trained but conventionally human soldier. His official nickname is simply “the Rookie”; the game does not provide him with a personal name, which helps preserve the character’s quiet, player-projection role.