Select Charlotte and equip her with the Str, Luck, Mind, Con, Int or All Boost. Then, call Jonathan as your teammate. Use a fully-charged boost with Charlotte, then become Jonathan and send Charlotte out (press A if your commands are set to the default). Jonathan will keep the Boost (+600 instead of the smaller boost (+30). Use this trick when Charlotte is not strong enough to kill a monster or when looking for rare items, such as the Medusa Whip, Eye Of Decay, or Summon Medusa spell.