A dream beast with the power to control dreams. It has a crystalline organ on its chest, known as the Guileba-Crystal, and emits dream beams from multiple red tentacles that emerge as it spreads its wing-like ears. Under Sweed’s control, it attempts to sever the bond between Yuma and Arc in a dream, asserting that the existence of the Arc, kaiju and aliens were all merely a figment of Yuma's imagination.

Guilebaku’s arms are held against its chest around the Guileba-Crystal, but they also have tentacle-like arms. Guilebaku extends and contracts these tentacle arms to release cutter-like light slashes (Guilebaku Circle Cutters). Similarly, in addition to the small ears on its face, it possesses wing-like ears that house attack-capable tentacles.

Guilebaku first appeared as a massive energy entity passing through a wormhole over Hoshimoto City and materialized as a kaiju. Using the power of the Guileba-Crystal, Guilebaku generates a barrier that envelops its body in a hemispherical capsule-like barrier (Guilebaku Dome Barrier), repelling the GDF’s attacks as it advances. When Ultraman Arc appears, Guilebaku traps him inside this dome-like barrier.