one of the multiple lead actors/actresses starring in a play, TV show, or movie;
e.g. He became close friends with his costar from that movie series.
(verb)
to appear in a play, TV show, or movie as a lead actor alongside other actors of equal importance, or to include a particular actor alongside others in leading roles;
e.g. She is scheduled to costar with her sister in a movie this summer. Many TV shows costar a mixture of well-known and lesser-known actors.