None of these are particularly universal, but with any luck a few of these will be helpful to you!
"And then--you won't believe this, cousin--but then around the corner comes some..."
- Goober
- Dork
- Nerd
- Fucker, asshole, etc
- Individual (the informality here is in the pronunciation, which is highly exaggerated, similar to, "In Dive Id You All")
- Wild child
- Loon, Looney-Toon
- Troublemaker
- Total stranger (ironic/sarcastic)
- Bosom buddy (also ironic/sarcastic)
- Twitter user, tumblr user, etc (note: usually insulting in face to face conversation; context will vary).
- Soul / Spirit
- New Character / New Dramatis Persona
- Stiff
- Customer (eg 'tough customer, cool customer, etc')
- Nongendered aesthetic term like goth, emo, hippie, etc etc
- Comically specific descriptor + object word, eg, 'graphic T shirt wearing, probably drinks warm soda looking nonsense' where 'nonsense' is the person.
Obviously some of these are unusual enough to need to be Delivered With Confidence if you don't want your story sounding weird, but fortunately that's only even relevant in spoken conversation, not Tumblr Dot Gov.
And in my experience, all of them are at least suitable in some kinds of speaking situation.
And, in the plural, you can usually just say "folks" or "people" and carry on, of course.