What is its function? Jonathan walked behind me.

Behind

Prepositions are words that introduce information to the reader. This information can include where something takes place (such as 'at' the store), when or why something takes place (such as 'before' dinner), or general descriptive information (such as the girl 'with' the cool tattoo). Either is used before the first of two (or occasionally more) alternatives that are being specified (the other being introduced by "or")."or is used to indicate a similarity or link with a statement just made.