The following article contains information that may be of great importance in the series, read at your own risk.
The whole fact that he accidentally killed Tem is made tragic by the certainty that all bears are now forced to take medications that limit their strength when they grow beyond 200 centimeters in height. These come with terrible side effects. Tem's friendship made Riz think he could manage without his pills. Riz remembers that Tem accepted him exactly as he was and remembers them hugging each other, before his memory screws up, after having Tem annihilated.
From the first scene of the manga, we know that Tem died cursing his own killer. Later, Riz admits that he was so happy that Tem saw through his exterior facade the "monster" that was inside, that he felt so much affection for Tem for revealing his true insidious personality that he altered his memories to make them happier, but it seems that the event has given him a taste for carnage. It has only advanced in recent chapters, to the point that he now sincerely believes that Tem wanted to be devoured and Riz eating him alive will make him his "best friend".
Riz has an extremely incredible strength, even when compared to other carnivores and has to regularly take force-limiting medications to make sure he keeps his strength under control. He stopped taking them in preparation for his big brawl with Legoshi.
He attacks Legoshi for investigating Tem's death and leaves before Legoshi can discover his identity. When he and Legoshi take Kibi to the nurse's office, Legoshi reveals that he knows that Riz is the one who killed Tem. They were about to have a fight to the death right then and there, but were stopped by Pina who decided to step in and tell that he now also knows about what Riz did, and blackmails him to leave them alone.
When Riz and Legoshi find themselves alone in the male drama club locker room, they decide to finish what they started. Riz decides to take Legoshi to the showers where blood stains will be easy to wash away. Due to the cleaning lady wondering why students are in the showers at a time when the showers are closed, they agree to duel at New Year's Eve under the Gazura Bridge.
When Riz arrives for the duel, he tells Legoshi that he killed Pina and smeared his blood on his hands and torso to make Legoshi angry, so he won't hold back. Legoshi's incredible sense of smell reveals to him that Pina is alive and tied up in a dumpster with a gauged arm. After several smashings against the concrete, Legoshi could barely breathe and move, so he tells Riz to take a break and hear what Riz has to say before Riz finishes him off. After having a heart-to-heart talk, Riz helps Legoshi get up, so they can continue the fight. Legoshi notices that Louis has arrived to witness their duel, so he takes off his jacket and throws it over Riz's head, so he doesn't see where he and Louis run off to.
After about 15 minutes, Legoshi returns, but this time in "beast mode" due to him eating Louis's right foot. Seeing that Legoshi and Louis have established a genuine carnivore-harbivore friendship, Riz's spirit is broken and declares Legoshi the winner of the duel. Meanwhile, Pina managed to escape and called the police, who arrive just in time. Riz gets arrested and Louis apologizes for being a terrible upperclassman and never trying to notice how lonely he was. Riz gets sent to juvie while Louis is sent to the hospital and Legoshi is sent to the police station.
Pina has been visiting Riz in juvie twice a month to see Riz's progress, as well as keep him up to date with everything that is happening.