Hetairoi (Companions) are a social and military elite, and fight as heavy cavalry using shock and mass to break enemy units.

Elite
Disciplined
Hardy

The Hetairoi, or Companions, were the elite noble heavy cavalry of the Diadochoi. They are an elite heavy cavalry that is second to none and arguably the best cavalry of the period. They fight with a degree of élan, discipline, and simple ferociousness that is matched only by the horsemen of Iberia, Karchedon, and the best heavy cavalry of the eastern nomadic peoples. They are armored from head to toe in iron plate (some still had bronze, but this was falling out of favor rapidly by 250 BC), reinforced with mail at key points. Their horses are barded with felt barding and often have bronze plates to protect their heads. Their helmets, with the two plumes, mark them on a battlefield. They are best used as heavy shock cavalry, able to decide many battles with a single charge. They are the second part of the hammer and anvil of Alexandrian battle tactics. They are armed with a xyston and a kopis, and are true masters of both weapons. If these men have any weakness, it is the front of a line of spearmen. Horses, not matter how well trained, simply do not want to charge into certain death. Nevertheless, these superb cavalry men are easily able to devastate even the toughest of foes with a well timed charge into their flanks or rear.

Historically, the Hetairoi were first created by Philippos, following the lead of the cavalrymen of Thessalia, to the south. Philippos went one better, armoring them to the teeth and giving them the xyston, a twelve foot long lance tipped with a large steel head. They were armored head to toe in bronze plate, with helms, cuirass, bracers and greaves. Alexandros often replaced this armor with lighter linen when they were traveling, but was quick to replace the bronze in any hard fought engagement. Alexandros added little to their equipment but trained them to ride their horses in such a way that each man was an expert horsemen as well as a shock cavalryman. The Diadochoi kept this mold, but added mail reinforcement to the bronze armor at the joints before replacing it with iron and added felt and lamellar barding to the horses after encountering enemy cavalry that used such equipment with a high degree of success. During the third century their usage declined with the Diadochoi’s ability to pay for such heavily armored men. They degenerated to such a point where only a handful were present at major battles, and were far from the war winning force that they had been earlier. Perhaps with a bit of luck and more funding, a commander could use them in their true glory once more...