Through an arcane ritual, a warlock-knight forms a bond with their squire: thereafter, the squire wears the armour, and the warlock gets the benefit of it. So any force (be that hand, weapon or environment) incident on the space above the warlock's skin where the armour would be, is magically transferred to the armour (in the same place and at the same angle of incidence relative to the armour) as worn by the squire. The warlock doesn't need to suffer the weight or awkwardness of the armour themselves.
The warlock and squire don't need to synchronise their movements or orientation, or even be on the battlefield together, although the only way the squire would be able to anticipate impacts would be by watching the warlock. Any blows which penetrate or bypass the armour injure the warlock, not the squire (it's as though the warlock is wearing an invisible, weightless set of (I presume) full plate), but the squire is affected by forces transferred through the armour.
What armour and combat techniques would these fighters develop to maximise their advantage from this ability? How much of an advantage would it give them over ordinary combatants?
If the squire dies, the bond is broken and the warlock is left in the middle of the battlefield with no armour at all, which is obviously a Bad Idea. So while the squires are considered subservient and ultimately disposable by the warlocks, they have to give at least some consideration to their welfare.
Setting is medieval fantasy, good metalworking skills but no gunpowder. Warlocks would have the financial resources to equip themselves and their squires with the best weapons and armour available.
Update some great (and hilarious!) answers here. To adjudicate on a few questions:
- Armour needs to be 'quite close' to the skin to transfer via the ritual, so no tanks or shields. But I can't see why spiky protrusions shouldn't function as weapons.
- The invisible armour wouldn't block light or heat flow to the warlock, but I can see the argument for it blocking airflow to the same extent as the real armour.
- How to deal with the conservation of momentum and energy is currently why the pile of used scrap paper in the corner is growing so rapidly :p The invisible armour retains the same absolute position relative to the warlock's limbs as the real armour is relative to the squire's limbs, so if you hit hard enough with a hammer that the armour gets pushed into the skin, then the hammer will be in contact with the warlock and is bound to transfer some energy/momentum. But for proper application of the Rule of Cool I think it should be possible for a warlock to stop a mace dead with a bare arm, even if that means a squire going flying behind the scenes :)