Rose

Rose sits behind Donnie in The Mystery of the Robot Rivalry.

There aren't any closed-captions indications as to what lines she speaks, and nobody says her name in that episode.

The Musical Mystery is more helpful because it shows she is sighing after playing the drums. By the end of episode Donnie also plays drums.

Rose is what she is called there.

Rosey is credited at the end of "Robot Rivalry" so that might be her full name. It is possible that the captions accidentally omitted the Y.