From Halopedia, the Halo wiki
Office of Naval Intelligence
Cox was a member of ONI Security stationed onboard Ivanoff Station during the Raid on Ivanoff Station.
Soon after Jul 'Mdama's Covenant faction began attacking Ivanoff Station, John-117 decided to destroy the Composer with a Havok nuke. The troops in the area were ordered to guard the Composer while the nukes were prepared. Cox's sergeant ordered him and Edwards to take the flanks.
He is likely named after Greg Cox, an environment art designer in Halo 4.