IN: Which person is he?
The boy ran from the cop because he saw the crime.
OUT: the cop

IN: Which person is he?
Brock understood the material better than Tracy because he was not paying attention.
OUT: Tracy

IN: Which person is they?
The NSA sent their agents in the field to retrieve intelligence because they need people in the field to do the field work.
OUT: The NSA

IN: Which person is they?
The Roman legion attacked the Persians because they had invaded.
OUT:
the Persians