R.U. Humerus

Voiced by the late Tony Randall, Ms. Frizzle's mechanic who appeared in "The Magic School Bus Flexes Its Muscles" and provided the M.I.K.E. to help the kids build RalphieBot. He insists on being called RU, and appeared to have some romantic attachment to Ms. Frizzle; throughout the episode, the two are seen teasing and mildly flirting with each other