Kagura is yet another member of the Sohma family who happens to be cursed by one of the vengeful spirits of the Chinese zodiac. She is two years older than Kyo and Tohru, and is the sign of the boar, so when she is hugged by a boy, she turns into a boar.
Kagura is a kind (albeit bad-tempered) girl, and violently declares that she is in love with Kyo. She states that Kyo promised to marry her, but Kyo reminds her that it was only after she threatened him.
Because she is so violent and the fact that Kyo does not return her feelings, he is not usually happy when she shows up. After Tohru acted very kind towards her, she wanted to be her friend, thus they are the best of friends throughout the series.
She also went with Kyo to his martial arts training in the mountains, and indeed she brought more trouble. They were constantly surrounded by boars (juunishi cused tend to attract the same animals that they transform)