A Core is an item that appears Super Smash Bros. Ultimate. They are used in summoning spirits and as an alternative to snacks. Cores can only be obtained by dismissing spirits, granting SP based on the spirit's class and number of slots it uses (for support spirits only) and a core of that spirit's type and class. Once obtained they can be sold, used in summoning, or be fed to spirits to level them up. Up to 99 of a spirit's core can be obtained; any extra cores gained while at 99 of that core will be lost.
As a snack
Just like snacks, cores will give a spirit experience but at the cost of some SP. Feeding a spirit a core that has a matching type to that spirit will give it 30% more experience at no extra cost.