Greybaum is a secret criminal organization in Custom Robo Arena. Their overall goal is to rule the world. They are seen (but not mentioned) early on in the game. They are referred to by Eddy repeatedly, but not revealed until the end. They secretly inserted their own men into NeoBrain to secretly create illegal parts and their masterpiece robo--Hadron. "Project Hadron" was Greybaum's secret project to rule the world. They originally wanted the hero to dive into Hadron, but once they learned of Liv's power-transfer ability, they swiftly changed their plans as Scythe put it: "Once we learned of the girl's ability, we changed our plans to take your battle data to make Hadron autonomous, and use the girl to give him a continuous source of power rendering him invincible!" They finally revealed their plan at the Robo Cup tournament after the hero infiltrated their base, disabled their defenses and defeated their leaders. Upon Hadron's defeat, he absorbed the power of a severely wounded Scythe (most likely killing him in the process) Hadron gained a consciousness of his own (most likely Scythe's) and the intent to rule the world he is then defeated by the Hero.
- Most main members have a necklace shaped like a crescent-moon with two spikes at the bottom, possibly making reference to the Z Syndicate's insignia from Custom Robo GC.
- Near the end all of them were knocked out by the hero upon their defeat and arrested. This is also nearly identical to what happened to the the Z Syndicate. (Albeit minus the memory erasing.)
- All of them have Battle Data in the Underground Arena except for Coyote.
- Most of the people in the Underground Arena look like and may possibly be ex-Greybaum members. One of them said he hates kids because one shut "their organization down."