From Halopedia, the Halo wiki
Cullen was a non-commissioned officer in the UNSC Marine Corps. In the summer of 2552, he was assigned to the 23rd Naval Air Squadron, and was stationed aboard the UNSC Pillar of Autumn. Cullen was the crew chief for Pelican Echo 419 under Capt. Carol "Foehammer" Rawley and Lt. Frye .
Battle of Installation 04
When the Pillar of Autumn arrived at Installation 04, the crew of Echo 419 immediately headed for their Pelican as their ship was hurling towards the surface of the ring world.
Cullen likely flew with Foehammer on all the missions during The Battle of Installation 04, and if so, was likely killed when the dropship was shot down by a flight of Banshees while they attempted to extract Master Chief Petty Officer John-117 from the due-to-detonate Pillar of Autumn. Although it is possible he could have survived the actual crash, which was not witnessed, he would certainly have been killed when the Autumn exploded a few minutes later, taking the entire Halo ring with it.
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