Summary[]
In v10, several of the party members have "natural immunity" to one element, giving them a free x1/4 multiplier against it. This is basically done through all the weapons they can equip (including the default) granting a hidden multiplier, and guarantees a final multiplier of x1/4 in that element.
Natural Elemental Resistances[]
Mario: None
Mallow: x1/4 Thunder
Geno: x1/4 Ice
Bowser: x1/4 Fire
Peach: x1/4 Holy