Runt works in Lab's Lab as Lab's assistant. If you look carefully, you might see Runt scurrying about making a latest invention or potion for the lab. He doesn't let his tiny size stand in the way for anything he wants to do, and he can climb nimbly in high spaces, or squeeze into tiny spaces. He doesn't really say much so you will never know what he is thinking about.
Runt has recently appeared in the Spooky Science mission since 2014 when you click the green candle 4 times, you will receive a figurine of him to put in your nest.