noun
leading actor; star performer
Specifically refers to the top-billed actor in a theatrical or performance context.
彼はこの劇団の立て物だ。
He is the leading actor of this theater company.
noun
leading figure; leader; central person
Extended from the theatrical sense to refer to a prominent person in any group or field.
彼女はプロジェクトの立て物として活躍した。
She played an active role as the leading figure of the project.
Compound of 立て (tate, 'standing' or 'prominent') and 物 (mono, 'person' or 'thing'), literally 'the one who stands out'. The exact historical derivation is uncertain, but it likely originated in traditional theater.