A 13 episode anime series put out by Bandai. Yagyu Jubei was Japan's greatest swordsman. On his deathbed, Jubei gives his loyal disciple, Koinosuke Odago, a heart-shaped piece of metal on a thong, the "Lovely Eyepatch"; he tells Odago that he is to give the eyepatch to the successor to Jubei's school of swordsmanship. When asked how he could identify the successor, Jubei says:

Plump.
Bouncy.
Big ones.

That's right, folks, plump, bouncy, and big ones ("Bonny boom boom bum" in the original Japanese). You'd think that a woman like that wouldn't be too hard to find in Japan, but Odago, through sheer force of fanatical will power, searched and lived on for 300 years until he finds 8th grader Jiyu Nanohana, an ordinary modern schoolgirl. Somehow Odago can tell that this particular plump, bouncy, and well built girl is "The One", and begs her to take the Lovely Eyepatch. Since she simply wants to be an ordinary teenage girl, she never uses the damn thing unless the villains are breathing down her neck, but since all of her teachers seem to end up being the villains, this happens quite a lot. When she dons the Lovely Eyepatch, she becomes Jubei-chan, an adult version of herself with a cool uniform and a kick-ass sword. (Although the eyepatch is a magical dohickey, and she goes through a transformation when she uses it, this isn't a magical girl show, since she doesn't do any magic; she just becomes unbeatable with a sword).

Ah, right, the villains. Thing is, when the original Jubei died, there was a ninja clan with a rival style of sword fighting that really wanted to defeat him, so they've been waiting for the last 300 years for his successor to show up so they can beat him/her. This clan also wants to take over the world, and they want to posses the Lovely Eyepatch before they start the conquering. This, I don't really get. Did the clan leader say "Once I have the Lovely Eyepatch, I shall be invincible! Muhahaha!"? *Sigh* Evil Overlords now-adays, they simply don't have their priorities straight.