While the third singular personal pronoun is[tag resources demonstratives_013] او [/tag] u ‘he, she’ (which is gender-free), in dependent clauses which are linked with the relative pronoun who,[tag resources demonstratives_014] آن [/tag] may replace the relative pronoun[tag resources demonstratives_015] کسی[/tag] ‘he who…, she who…’.

Compare the following two sentences:

[tag resources demonstratives_01].انسان واقعی کسی است که به دیگران خدمت می کند[/tag]
“A real human being is he or she (one) who provides service to others.”
(i.e., “He is at other human beings’ disposal.”)
[tag resources demonstratives_02].انسان واقعی آن است که به دیگران خدمت می کند[/tag]

Note that both are correct.