Mugann

As the human population on Earth reached one million, the Anti-Spiralized Nia began to mentally materialize these virtual weapons out of thin air that start destroying everything with high-tech weaponry, such as a massive laser that obliterates anything it makes contact with. After the first one appeared, these mecha were codenamed Mugann (from 無顔, meaning "faceless") by the human forces, because it does not have a face. They're three types of Mugann that appeared over the series, the soldier class, the black and green Jokyu-Mugann that acts in deploying and coordinating the attacks of multiple Mugann units, and the two Giant Cage Mugann that attack by separating into smaller pieces and reforming around the target, enveloping it and attacking from all sides or combine into a larger Mugann unit that mostly attacks using its body as projectiles before being destroyed by Arc-Gurren Lagann.

All three types have almost impenetrable energy shield which can deflect bullets and other solid weaponry, however, its defenses and attacks are not enough to stand up to the Gurren-Lagann, which managed to defend against its attacks and drill through the mecha during its initial appearance with the Giga Drill Break. However, unlike normal Gunmen that simply explode or break-down when dealt a fatal blow, these mecha turn into small explosives that devastate anything nearby, causing almost as much damage as the Mugann itself can cause. This is a result of the mecha's unstable molecular structure which also enables it to easily teleport itself through long distances.

A technique created by Simon to effectively defeat the Mugann without causing damage is by harnessing their lasers' power using the Gurren-Lagann and delivering it back to them all at once, destroying their body, and then shooting down every single explosive they left with the Grapearl force's handguns before they fall into the ground. However, it's also known that this measure could be troublesome against larger numbers of Mugann. Later, Leeron creates a technique where Grapearls can both penetrate the Mugann defenses and create a shield around it, meaning the Mugann can both be destroyed and have its small bombs rendered harmless.

When Nia was recalled, the Mugann stopped attacking Kamina City.

Trivia
The Mugann are the only 3D-animated mechas used in the series.