noun
Refers to the main role or central figure in a play, film, or by extension any undertaking.
彼はその舞台で立役を演じた。
He played the leading role in that stage production.
このプロジェクトの立役は彼女だ。
She is the central figure of this project.
主役 is the more common word for 'leading role' in modern Japanese; 立役 is somewhat more traditional or theatrical.
Compound of 立て (tate, 'standing' or 'main') and 役 (yaku, 'role'). The exact historical derivation is uncertain, but it has long been used in kabuki and theater contexts.