Hi!
In preparation of a live lesson I was confronted with the words “niemand” and “keiner”. As a native speaker I wasn’t really sure about the difference between these two. I actually didn’t even know there was a difference 
So here are some “rules” when to say which word, even though you can actually say both in, I think, almost every sentence (please tell me if I’m wrong).
“Niemand” is the negotiation of Jemand -> Ist jemand hier? - Nein, hier ist niemand.
“Keine/r” is the negotiation of ein/e -> Ist ein Arzt hier? - Nein, hier ist keiner.
“Niemand” is only used when it comes to people, but “keine/r” can also be used for things or animals
For example:
-
Kein Hund ist so süß wie meiner! (there’s no dog cuter than mine!)
-
ich habe noch keinem was davon erzählt (I didn’t tell anyone about this)
-
ich habe noch keinen Termin (I don’t have an appointment yet)
-
ich sehe hier keine Kinder ( I don’t see any children here)
-
ich kenne niemanden der so witzig ist (I don’t know anyone who is this funny)
-
ich habe noch niemandem was davon erzählt (I didn’t tell anyone about this)
“Niemand” can only stand alone in a sentence" - Niemand kann bestreiten… - No one can deny…
“Keine/r” is usually used with another word: Hier sind keine Kinder erlaubt… Children aren’t allowed here…
So the only significant difference is, that “niemand” can only be used when you’re talking about people (in general).
“keine/r” is used when you’re talking about things, animals or specific people (like doctors, men, women, divers…)
Hope it helps to clarify some things. 
. This is very useful, as a learner I always struggle with the use of “keine/r” 
.