Characteristics of Protists. Protists are eukaryotic organisms that cannot be classified as a plant, animal, or fungus. They are mostly unicellular, but some, like algae, are multicellular.Kelp, or 'seaweed,' is a large multicellular protist that provides food, shelter, and oxygen for numerous underwater ecosystems.esson Summary. Kingdom Protista contains the protists, or all the organisms that do not fit into the other kingdoms of life. Protists can be heterotrophic or autotrophic, mobile or immobile, single-celled or multi-celled, solo or a member of a colony.