Learn German: Ich kenne hier niemanden, außer dir. - I do not know anyone here, except you.

The preposition außer always requires the dative case. Even though the sentence's direct object is in the accusative case (niemanden), the word immediately following außer must take the dative form. Therefore, the accusative dich becomes the dative dir.

Why does niemand have an -en ending here?

The word niemand (nobody) is the direct object of the verb kenne, so it must be in the accusative case. While you will sometimes hear native speakers use niemand without an ending in casual speech, the formally correct accusative form is niemanden.

Why is the verb kennen used here instead of wissen?

In German, there are two main verbs for the English concept of knowing. You use wissen for knowing facts or information. You use kennen for being familiar with a person, a place, or a thing. Since the sentence is about knowing people, kennen is the required choice.

Is the comma before außer mandatory?

No, it is optional here. When außer adds an exception at the end of a sentence, you can use a comma to indicate a slight spoken pause or to treat the phrase as an afterthought, but the sentence is completely correct without it.