eager

One who is industrious and enthusiastic (perhaps annoyingly so). Give that big project to the new girl, she's a real eager beaver. Aren't you an eager beaver, getting to the office an hour early.

someone who is very enthusiastic; someone who works very hard. New volunteers are always eager beavers. The young assistant gets to work very early. She's a real eager beaver.

someone who works very hard and is very eager to do things Who's the eager beaver who came in at the weekend to finish this work off?

An exceptionally zealous person, one who habitually takes on more tasks or works harder than others. For example, Bill is a real eager beaver, always volunteering to stay late. This expression became especially popular during World War II, applied to recruits anxious to impress their commanding officers by such behavior. [First half of 1900s]