When we first meet Usagi, she seems like your normal teenager: late for class, voracious appetite, and very boy crazy. But within a few moments, we learn of her compassionate nature when she saves a cat in trouble. From this moment on, destiny and fate take control of Usagi's life. We learn she is not *just* a normal teenager, but Sailor Moon, champion of love and justice. She is also revealed to be the Moon Princess, a future Queen of the earth, and one day, the hope, and light of all the universe.
At first, Usagi does not take her job as Sailor Moon very seriously, but she soon learns how important she is to keeping the world safe, and saving lives. One of the biggest factors of her increasing maturity, is her relationship with Mamoru, her one true love. Their love was forged back when both were Princess Serenity and Prince Endymion of the Moon kingdom. Reunited on earth, they struggle to keep their love alive. Usagi's desire to be a better girlfriend, and one a day a bride, to Mamoru motivates her to be more mature, to study harder, and to be a better fighter.
Realization of her powers, and her love for Mamoru, propelled Usagi to face her fears, her weaknesses, and battle evil, often at the loss of her own life. This shows great maturity, because once, Usagi would have rather hid in her room than face danger. The best thing about her, though, is that she *does* face danger, even when her spirit is broken, as when she and Mamoru broke up. Despite her tears, she did what was needed, and it showed her strength, courage, and maturity.
In the anime, Usagi takes longer to mature than in the manga. Her reluctant, 14 year old persona undergoes tremendous shocks, more shattering than any female her age should have to face. Usagi has to be a leader, a fighter, a lover, and still try to be her own unique self. How many of us could say we'd still be sane and strong, after going through all of that? Usagi is just THAT strong a character. She undergoes so much in a short time, yet rises to every challenge, and defeats evil. Especially at the end of the series, defeating Galaxia and bringing peace to all.
Beautiful, strong, and powerful, Tsukino Usagi is one of the most amazing characters of anime ever, and will remain so, forever.