Ruler and J-ruler cards have no printed cost, so their total cost is always treated as 0. Thus, "Charlotte, Wielder of the Sacred Spirit" can never be the target of an ability of an opponent's ruler or J-ruler as long as its Barrier ability is applying.
"Barrier (Total cost equal to or less than number of cards in your hand)" means that this card cannot be targeted by:
1) Spells, controlled by players other than Charlotte's controller, with total cost equal to or less than the number of cards in her controller's hand;
2) Abilities with sources that have total cost equal to or less than the number of cards in her controller's hand and that are controlled by players other than Charlotte's controller;