Productive suffix that attaches to nouns or verb stems to form agent nouns or role nouns. Common in compounds like 医者, 学者, 読者.
He is a famous scientist.
This book is loved by many readers.
In the Edo period, 者 alone could refer to a geisha or a courtesan.
者 is also a suffix for people, but tends to sound more formal or role-specific (医者, 科学者), while 人 is broader and used freely for nationality or general types (日本人, 芸能人).
Standard kanji form for the suffix and the archaic noun.
From Chinese 者 (zhě), a nominalizing suffix. In Japanese, it became a productive suffix for agent nouns and was also used as an archaic standalone noun for certain professions.
author; writer
the latter; the second of two · successor; descendant; those who come after
hero; brave person; man of valor
believer; follower; adherent · fanatic; superfan; fanboy
person concerned; interested party; stakeholder
actor; actress; performer · clever person; man of wits; cunning fellow
both (of two); the two; both parties
the former; the first (of two)
another person; others
king; monarch; ruler · champion
winner; victor
wise person; sage
eligible voter; elector; constituent; the electorate · holder of a right; rightsholder · person with political clout; bigwig
instrumentalist; player
messenger; envoy; emissary
batter
writer; author
loser; defeated person; the defeated
weak person; the weak; vulnerable person; disadvantaged person
strong person; the strong; the powerful
warrior; samurai