indispensable person

See: key man
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References in classic literature ?
About herself, Sonia wrote that she had succeeded in making some acquaintances in the town, that she did sewing, and, as there was scarcely a dressmaker in the town, she was looked upon as an indispensable person in many houses.
Franklin Delano Roosevelt once said that there is no indispensable person.
2) "Are you the indispensable person in your company?