Calculator is a Job from Final Fantasy Tactics. A high-level magic class, the Calculator uses Math Skill which can cast most magic for free and without a Charge Time, assuming they already know the spell and they can do the right calculations for it. Calculations are done by choosing a condition, like Level, then choosing a number, like 5.
With those conditions, Calculator will cast a selected spell targeting all units (allies and foes) whose level is a multiple of 5. To counter their powerful abilities, Calculators suffer from having fairly low Magic attack power among the mage jobs and the lowest Speed rating in the entire game, and other units may take many turns before the Calculator's even comes up.
An easy way to counter this is to send the unit on errands. Another viable tactic to raise JP for them is to have their secondary action ability set to Dance for female or Bardsong for males; this will increase the turns they gain JP since they'll perform their Bardsong or Dance action more often than their turn will come up. It costs 4,200 JP to master this Job.