Souchirou Nagi and Bob Makihara are two delinquents who arrive at Toudou Academy with the objective of ruling over the school with their strength. But on their first day, they encountered the Jyuukenbu, a martial arts group of the school. Members of the group, Maya Natsume, her sister Aya, and Masataka Takayanagi easily overpowered Souchirou and Bob. Seeing no other option the two decide to join the group and receive training to become stronger.
The anime focuses a lot on Nagi and Aya. Aya wanting them to have a relationship and Nagi confused and discovering his feelings amidst all the fighting. Another key character not mentioned above is Aya's late brother Shin Natsume. He is a key part of the plot as a lot of the characters at the school have connections with him and become involved through fighting because of heated feelings about him and various misunderstandings. This also works in reverse as Maya and Aya also involve some of the people in the school due to these feelings and misunderstandings.
Aya is a first year at Toudou Academy. She is very similar to her sister in appearance and build. Since meeting Souchirou Nagi Aya is determined to become his wife this causes tension between her and her sister as Souchirou loves Maya. Aya loved her brother dearly and having not been old enough to fight with him knows little about the circumstances of his death. This causes her to have a lot of built up anger that is released when she finally finds out what happened to him. Aya possessed the family's supernatural power called the Dragon's Eye.
Souchirou is a first year student and self proclaimed delinquent. He is the heir of the Demon Exorcist family. His family's supernatural power is called the Dragon's Fist, which gives them the ability to take supernatural powers from others and use it as their own. His fighting reflects that of a street punk and requires refinement which we see him doing throughout the series making him stronger up until the point that he's almost unbeatable.
Maya is a third year student and the current leader of the Juken Club. She is very skilled in various martial arts, but does not possess the Dragon's Eye like her siblings. For this reason her father entrusted her with the cursed sword Reiki. Early in the series, she would use a body manipulation technique to revert to herself into her childhood form to conserve her ki. She occasionally shows slight feelings towards Souchirou and like Aya is very angry about the circumstances of her brother's death.
Masataka looks like an average high school student, but he is surprisingly a powerful martial artists. He is always a formidable fighter, but he becomes much more dangerous when he is serious or angered. He is a very easy going and jokey kind of guy. However, his inner nature is perverted, even though he dislikes this part of himself. Masataka falls in love with Aya Natsume during the first episode and feels disheartened by her love for Souchirou. Throughout the whole series he is in love with her and is completely oblivious to other girls and their crushes on him.
Bob is a first year student who is of African descent. He has been friends with Souchirou since elementary school because of this throughout the series you see them hanging out and fighting together. He is athletic and practices the Afro-Brazilian martial art of Capoeira.
Maya and Aya's deceased older brother. He once was the president of the Executive Council and the founder of the disbanded Team KATANA and the Juken Club. He possessed the family's supernatural power called the Dragon's Eye, which allowed him to have such abilities as precognition, clairvoyance, and clairaudience. Because his power was awakened when he was still a preadolescent, he could not control his powers and was driven insane by them. In flash backs we are shown his love for Aya and Maya and his insanity that is reflected in his fighting technique.
Sources: Myself, myanimelist.net and Wikipedia.
If you watch this, like it or want more that are similar I suggest you watch History's Strongest Disciple Kenichi which is similar as it's a fighting anime which contains a lot of comedy.
I don't think there are many spoilers in there but if there are I apologise. The first thing I saw of this anime was the opening song: http://www.youtube.com/watch?v=74fP4waPN9Q&feature=related.
And this picture:
And I thought wow! How cool does that look. And I was right.
Hope you guys enjoy it. :)