الَّذِي كَانَ مَعَنَا أَمْسِ رَجُلٌ عَالِمٌ
The one who was with us yesterday is a learned man.
The relative expression «الذي» functions as the subject, «كان معنا أمس» is its identifying clause, and «رجل عالم» is the predicate. The hearer may not know the person's name but remember yesterday's encounter; the clause bridges that informational gap.
The clause is not merely an ornamental compliment. It is the practical route to identifying the referent. If the hearer has no recollection of the encounter, this particular relative description will not serve that identifying function.