The ADR-09: Gatekeeper, also known as the "D-Reaper Ball", is a Base Defense Agent. It is formed to protect the D-Reaper's core, the "Kernel Sphere".
This Agent was formed around the Kernel Sphere to protect it. It was assimilated back into the D-Reaper, forming the face of Mother D-Reaper in Episode 49.
35mm α Houkai Sokushahou (35mm α崩壊速射砲, 35mm α Houkai Sokushahou? lit. "35mm α Decay Rapid-fire Cannon") #ADR