Lester the Rat (played by Mark Ritts; voiced by Salvador Delgado [Mexican Spanish] and Carlos Silveira [Brazilian Portuguese])
Becoming the Mask: In early episodes, he resented having to wear the rat costume as he was a professional actor whose agent booked him a crummy role on a kids' edutainment show. As Flanderization kicked in, he apparently began to actually identify himself as a man in a rat suit as if it were his species(as well as the fact that friends and family members inexplicably became people in rat suits as well).
Awesome Moment of Crowning: The last segment of Josie's last episode shows kids how they can do their own research (using the question "How do fireworks make those neat colors?" as an example). Beakman points her to resources, but Josie does most of the legwork and provides the final answer herself.