Gabrielle (Xena: Warrior Princess)
Gabrielle starts the series as a young girl who more or less stands behind Xena, and relies on Xena to protect her. In the course of 10 episodes, she acquires a fighting staff from the Greek Amazons and is a decent fighter with it shortly after. Another 10 episodes later, she displays immensely impressive skills with the staff, taking on numerous soldiers single-handedly, in the attempts to rescue Xena's presumably dead body. Probably what is most impressive, is that two episodes later she blocks an arrow in flight with her staff.