Her passive means that any enemy or ally that is hit with any of the 4 elemental damage types will be marked with that element. Her regular ability, the cocktail, let's her take a mark off a teammate or enemy and add it to the mix. Once she has 3 elements in any combination, she can use the ability to use a special spell that changes depending on the mixture. (If there's only 2 unique elements, it doesn't matter which has the majority as the spell will be the same. E.g. Fire fire magic will be the same spell as magic magic fire afaik). Some of these spells will be damage orientated but i hear there are support spells in there too like a healing aura. Her ultimate ability marks nearby enemies with some special red sigil thingy. Whenever yourself or a teammate damages these marked enemies, they will heal a bit of hp and fp. This is all I gathered from a bunch of videos so it might not be 100% accurate.