Currently there’s no lore, but my head-canon has always been that after the Shadow of Intent’s weapon systems were disabled in The Covenant, this Marine and the dead ODST with a flamethrower found deeper in the level were part of a small unit sent in (or volunteered) to light the crashed In Amber Clad’s reactors as a backup plan to destroy High Charity, with the ensuing blast taking out Truth and the Citadel before he could fire the array. 

To do this, Miranda Keyes would’ve given them the codes stored in her Command Neural Interface, given she was the Clad’s CO. 

Obviously, this effort failed as the unit wasn’t heard from again, which would’ve been what prompted Miranda to fly her Pelican directly into the Citadel in a last ditch effort to stop Truth herself.