The Palace Guards were employed by Zenoheld and Professor Clay at the Vexos Motherpalace.
They first appear In Facing Ace during Mira's flashback alongside Prince Hydron watching the brawls.
They are first seen in Six Degrees of Destruction.
In Ambush, they chased Dan Kuso and Mira Clay when they snuck into the Motherpalace.
In An Heir To Spare, a Palace Guard informed Hydron that Zenoheld requested his presence. Later they bring the The Agonizer on Zenoheld's orders to punish Hydron for his failure.
Given that none of them were seen inside it in the final battle against the brawlers, it's likely that they all perished when the Alternative was launched from the Motherpalace.
Other guards appear different in the Vestal cities.