Scientist #1 is a scientist who is always conducting experiments on the beavers.
Although Scientist #1 says he and Pete are scientists, he's not exactly the brightest, or at least he's always able to get outsmarted by the beavers.
In the first episode he appears in, he said that if scientists like them couldn't find a cure for the disease stinky toe no one could. However, Dag finds one for Norb just in time using their own equipment.  They were one time environmentalists and thought Dag was the last of the "Great Horned Beavers". 
Scientist #1 has brown hair, a big cleft chin, and wears glasses, a light yellow shirt, a reddish-brown tie, a white lab coat, tan pants, and black shoes. He later added a name tag with "1" on his lab coat. 
He owns a Honey Bunny boom box, which is kept in his lab coat. 
The Angry Beavers Wiki has a gallery for Scientist Number One