girl Friday 
noun
A female assistant, especially a junior office worker.To them, she had been an insider girl Friday for powerful social conservatives....
- When I come back I'll be looking for a girl Friday to sail around the world with me.
- It is, after all, a story of an artist struggling for recognition within the cultural establishment - a photographer working as girl Friday for a cold-hearted gallery owner.
Origin
1940s: on the pattern of man Friday.