Ancient and powerful humanoid constructs that were built by an ancient race called the Senshi. Gaia is the only surviving Sacred Mechalord, and was unearthed intact (although immobile) by the Shtrayu Empire. It was mainly used as a decoration, before the arrival of Kenshi triggered a response from it.
It was revealed that the Senshi civilization was completely destroyed by Gaia, and later on it is revealed that Babalun's father put Gaia's Core Crystal in his body when he was young.
After Gaia had been revived with Doll inside of it, Kenshi faces off against them, and to everyone's astonishment, was able to fight with them on equal terms.
The tables are later turned when Babalun merges with Gaia, and begins to overpower Kenshi. The latter decides to go all out while Gaia continues to regenerate.
Kenshi later hears Doll/Mexiah asking him to save her from Babalun/Gaia. In the midst of the battle, Kenshi goes berserk and generates a Light Hawk sword, causing him to destroy the main core of Gaia and save Doll/Mexiah, both of whom have fallen in love with Kenshi.
Babalun later emerges from the scraps, vowing to kill Kenshi, but Ulyte intervenes and kills Babalun, at which point he collapses and reveals his true identity as Rea Second.
At the end of the story, it is hinted that Dagmyer became infected by Gaia's core jewel.