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The one who came to visit was the principal himself.
Literal
Came-to-visit [one-の] [topic-は] principal [that-その] person-[polite-でした]
The cleft structure: [relative clause]+のは + [focused element]でした translates roughly as 'the one who X was Y'. その人 ('that very person') adds emphasis — 'the principal himself', not just any principal. A good example of topicalizing a relative clause to foreground its subject.