For me it usually starts with either a core idea I think is interesting, or a funny idea for a joke/interaction, then I build a character around that.

So like having the spitting image of a final villain being a supporting NPC(Wily), or someone who constantly sees death being chipper and kind instead of jaded(Polly)

Sivrit started with a joke about him being mad at someone for looking like a draconid when using battle bond, and that turned into this idea of a proud and honorable draconid warrior with anger issues.

Another example is how Salvador started as a parody of the "faceless hentai guy" including being gigantic, but he's both super nice, kind of a loser and a virgin(until Ranae).