co-star
noun
Definitions
A person who shares star billing
Example: The once famous actor objected to his costar having a bigger dressing room.
A person who slightly lacks the status to be considered a star
Example: Alas, always a costar but never a star.
verb
Definitions
To perform with the billing of a costar.
Example: People thought his career was over but now he will get to costar on Broadway next month.