Kuwabara, as most people call him, is the second-toughest kid in Sarayashiki Junior High School and is always trying to usurp Yusuke Urameshi's position as the toughest. In the manga(chp.1) he says he never lost a fight until he met Yusuke. He leads a gang consisting of himself, Kirishima, Okubo, and Sawamura (In the English dub, Komada). He has a soft spot for kittens (including his own, Eikichi) and an iron code of honor. He develops a minor crush on Botan when he first meets her, but later harbors intense feelings on the ice apparition Yukina, but is unaware that she is Hiei's sister. Kuwabara has a tough older sister named Shizuru Kuwabara. She and Yukina are the only characters known to call him by his given name. In the anime, Kazuma and Shizuru's parents never appear and are rarely, if ever, even acknowledged. His father does makes a brief appearance in the final volume of the manga to reveal that Yukina is going to be living with them and appears to be aware of the Reikai, etc. As the show progress, Kuwabara's and Yusuke's relationship evolves from rivals to best friends, although the two still disagree constantly. Kuwabara and Kurama are quite cordial with each other, but Kuwabara shares no love for Hiei, and both will readily mention their hatred of the other, although in truth deep down they are friends. Kuwabara's most unusual ability is his sensitivity to Reiki, or Spirit Awareness. He and his band of friends call this "the Tickle." Kuwabara has the ability to sense when ghosts and the paranormal are around, can see spirits and other demons, and can predict motives of others. Kuwabara can also create a blade out of his own spirit energy (known as the Rei Ken, or Spirit Sword). Although he first needed a piece of wood as a physical entity to create it, Kuwabara was able to practice and be able to create his own after one month (six months in the English anime). During his training, Kuwabara's sword is able to extend and shrink to his whim, and he can even create two, one in each hand. Kuwabara, unlike the rest of his teammates, can be reckless, but is extremely loyal to his friends and has a very strict code of honor. He would do anything for his friends and for love but can be quite crafty in battle when he needs to be, and he is widely considered as the nicest of the four.

Mukuro Rokudo is one of the main characters of Reborn! and is also the first season's primary villain. Portrayed as the series' first primary antagonist, Mukuro Rokudo is introduced as a 15-year-old Mafia criminal who is the leader of the Kokuyo Gang, which consists of heinous criminals whom have recently escaped from prison. However, later on in the series, he takes on a more supportive role for the series' main characters, becoming somewhat of an ally rather than an enemy, though he prefers to not consider himself as one of their allies. Despite often being shown with a playful smile, Mukuro is seemingly apathetic to the suffering of others. Appearing as the series' first antagonist, Mukuro is not averse to disposing of those who get in his way, and is not easily intimidated, usually speaking to others in a very direct and arrogant manner. He cares little for others, and simply considers people to be "toys" or "tools" he can sacrifice in order to get to what he wants. Mukuro is implied to have a strong dislike for the entire Mafia underground, due in part to the experimentations he suffered as a child at the hands of the Estraneo Mafia family. When Mukuro killed his captors, he offered Chikusa and Ken, who were also victims of experimentation, a chance to join him in his quest to destroy the world. However, not only does he want to take revenge on the Mafia by destroying it, he also wants to cleanse the world of its filth by invoking universal suffering through a world war.