Seems like my guess about Ryuunosuke's gender was dead wrong. If this were a more harem-oriented show, I would have been right
Perhaps Rita herself aspires to be a great artist but her talent is not even close to that of Mashiro and she's crushed that Mashiro, who has the talent that she wishes for, chose another path? I don't think Rita is merely concerned about 'what's good for' Mashiro, that's my guess.