Before you stands, unarguably, a really -
really attractive young woman! Albeit a little oddly dressed...
Mami the Magical Girl!
Well, believe it or not, there's a reason she's dressed like that! Even if it is never explained why she needs to be dressed this way, the young Mami Tomoe here is actually a Puella Magi - a Magical Girl. A Puella Magi, basically, is a young woman who's had a normally impossible wish granted, and is given powers to destroy Witches which lay curses on humans across the globe.
There's much,
much more to it, but - that's all you'll get to know for now.
Every Puella Magi is given specific powers - and Mami's are quite strong. She is able to summon a seemingly endless amount of Percussion-lock Rifle's, as well as an enormous Percussion-lock Cannon to finish her foes. These muskets are surprisingly made out of ribbons cast forth from Mami - who can also ensnare her foes in them!
Mami's personality
Mami is incredibly sweet. She has a strong sense of justice, which has helped her become a powerful veteran at her profession - at the same time, however, since most Puella Magi don't like to team up with others, this has left the parentless Mami incredibly lonely. She exudes a calm, calculating and confident aura in battle - while maintaining an "older sister" vibe to her peers - However, the young woman is actually incredibly lonely, treating the Witch-hunting like a game so that it seems less scary. She openly admits to a friend that she cries all alone from her fear, some times.
Unfortunately, Mami seems to have chased a particularly powerful witch into a vast, bizarre Nexus - getting lost within it.