solo

When a post only features one character.

What exactly counts as a character?

Someone physically present within the scene, in whole or in part (including things like disembodied hands).
Someone interacting with the scene whose likeness is visible (e.g. on screen in a video call, a cutaway, etc.).
Someone present in static media, such as a photograph, when only static character likenesses are present (Example).
Multi-headed characters are counted only once. Do not count each head as a separate character.

Except if the following is true:

They are tentacles. Those only count if they are attached to a visible creature (octopus, tentacle monster, etc.). That is, a picture of just tentacles would actually be tagged with zero pictured.

Each image in multi-image posts is tagged separately, which means that it's possible (for instance) to have both solo and duo tags on the same post.

Multiple occurrences of a character in a sequence, model sheet, etc are only counted once. For instance, a single character drawn in multiple poses is solo.

Comic pages are tagged cumulatively by scene. For instance, two characters talking to each other is tagged as duo, not solo, even if drawn in separate panels.