The Ark's Retriever Sentinels are among the most powerful Forerunner drones, their primary purpose being the mining of the Foundry in the creation of Halos and Ark maintenance. However, like all Forerunner drones, they can also be used to defend the Ark by turning its sentinel lasers on anyone who threatens it.
The Retriever Sentinels played an important part in repairing the Ark after the events of 2553 in the final battle between the UNSC and the Covenant. UNSC scientists at the Henry Lamb Research outpost reactivated the dormant retrievers and watched in awe as the Ark was restored to its former glories. Sadly, war has once again come to the Ark, but the Forerunner drones built to protect it will stand fast and push back against any and all invaders.