While the third singular personal pronoun is او u ‘he, she’ (which is gender-free), in dependent clauses which are linked with the relative pronoun who, آن may replace the relative pronoun کسی ‘he who…, she who…’.
Compare the following two sentences:
.انسان واقعی کسی است که به دیگران خدمت می کند
“A real human being is he or she (one) who provides service to others.”
(i.e., “He is at other human beings’ disposal.”)
.انسان واقعی آن است که به دیگران خدمت می کند
Note that both are correct.