They are attached to a plant or animal substrate (thus epiphytic or epizoic), via a stalk or a circular pad.

The cells have diverse shapes, ranging from fusiform to club-shaped or spherical, and are variously straight, hooked, or S-shaped.

[3] The genus Characium is similar to Characiopsis, a type of algae in the class Eustigmatophyceae.

The two genera are very similar and have been confused in the past;[5] however, Characium produces starch to store excess carbon and thus stains brown to purple in Lugol's iodine solution, while Characiopsis does not.

These include Deuterocharacium, which differs in having chloroplasts without pyrenoids,[7] and Characiopodium, with multinucleate mature cells.