'Cast opposite Ms X ' = given the role of the character who is the lover, mate, companion, etc., of Ms X's character, if the roles are of roughly equal importance.
Ms is the current abbreviation in the States that replaces Miss and Mrs. It is pronounced /miz/.
In a play or a movie, the actors make up the cast. (noun) The director casts (verb) the actors he wants in the various parts. The female and male stars are cast (past participle) opposite each other.