Kuwabara, long time rival of Urameshi Yuusuke, is one of the four main
characters, and best friend of Yuusuke. He has his most famous phrase
"I, the great Kuwabara Kazuma, will [insert lame threat]. OR
"I, the great and honourable Kuwabara Kazuma will not become/do/degrade myself to [insert something such as 'fighting women']
He surpasses all other characters in any anime I've known (besides a few from Slam Dunk) in the field of providing comic relief. Not only is he supposed to be ugly to the extreme, he is able to be seriously beaten up over stupid and hilarious actions or comments, and can twist his face into SD or entirely amusing expressions. He is indeed the hero in the saga of Sensui, almost the main focus I would presume, except that Yuusuke stole the camera in the end by dying and being reborn as a demon. Possessing the 2 dimension sword, Kuwabara is able to fit in as the most important character in the whole thing about openning a gap in the dimension between Ningenkai and Maikai.
Kuwbara is not entirely a great fighter compared to his three associates, who can move faster than the naked eye can see. However, he is extremely tough, and works his power through emotion. In the Ankoku tournament, Toguro Ani lost to Kuwabara when he so graphically enacted the death of Genkai (real stupid mistakes of villains. They always tend to really upset their opponents, giving them the advantage). Rage and anger inside Kuwabara was able to block Ani's piercing fingers, which no one seems to be able to do till that time. Against Sensui, when Yuusuke was fighting alone, and they were all trapped inside Itsuki's dimension and knowing that Yuusuke intends to die, Kuwabara is so overcome with passion and grief and anger, that he manages to control his 2 dimension sword and escapes with his group. I'm sorry to offend people who do not like Kuwabara, but he is an absolute legend.