From ErfWiki

Speculation

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 Knob^Erf-b1-p079. We also know some were in use by the Royal Crown Coalition, as some were in Gobwin Knob's decrypted army^{[] book link name not found Please Add to Template?px=%2F] book image name not found Please Add to Template Erf-bFirst_intermission-p4]}.

Stanley started out as a Piker infantry under the command of King Saline IV of the Plaid.^Erf-b1-p079

Further Speculation

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.