الَّذِي كَانَ مَعَنَا أَمْسِ مُسَافِرٌ
The person who was with us yesterday is a traveler.
A relative noun identifies its referent through a relative clause (sila). A form such as “the one who” becomes informative when the attached clause identifies the person.
In the example, «الذي» is the relative noun, «كان معنا أمس» is its clause, and «مسافر» gives the predicate. The referent is recognized through a descriptive event rather than a proper name or pointing.