The organism is single-celled is the characteristic that would be most helpful in classifying the organism as a protist.
Protists are microscopic, eukaryotic (cells have a true nucleus), unicellular (or single celled) organisms including protozoans, slime molds and some algae; historically grouped into the kingdom Protista. Some protists have flagella or cilia used for locomotion and they reproduce both sexually and asexually. Protists can be found in any environment that contains water. They can feed heterotrophically or autotrophically.