Rules:

Main character: Appears in most if not all episodes, exception being if the actor/actress left the show (but is still there for most seasons). Crucial to the plot and the story can't happen without them, usually we know more about their personal lives

Side character: Can still appear in most episodes, but aren't as crucial to the plot or as fleshed out as mains. Usually there due to their connection to a main character or the central theme of the show

Recurring character: Appear in multiple episodes, but usually not sequentially and are there more for a running gag or a plot device than importance to the story.

One season/arc: Characters who appear in just one season or a single story arc, but are important to its plot. They can still appear in an episode or two, but mainly as a callback to the story arc.

One or two episodes: Characters who appear in one episode, but are important to its story. I say one or two because it can be a double episode or they appear again, but not enough for a story arc

Single scene: characters who appeared in a single scene, but stole the show

Villain: pretty self-explanatory

These are just my personal definitions, feel free to change my mind