A Piker is a type of Infantry Unit (as in "piker-class infantry").Erf-b1-p077a Presumably, it uses a pike as its primary weapon.
Pikers are used by Gobwin KnobErf-b1-p079. We also know some were in use by the Royal Crown Coalition, as some were in Gobwin Knob's decrypted army.
Stanley started out as a Piker infantry under the command of King Saline IV of the Plaid.Erf-b1-p079
Pikes and spears are traditional anti-'mount' units in most games, and weak to archers and swordsmen. It could be assumed that they carry a bonus to mounted units.