noun
Originally a kabuki term for the main actor; now used broadly for the star or lead in any performance.
彼はその舞台の立役者として大成功を収めた。
He achieved great success as the leading actor in that play.
noun
leading figure; central figure; key person
Figurative extension: a person who plays the most important role in a project, organization, or event.
彼女は会社の再建の立役者だ。
She is the key figure in the company's reconstruction.