Ren is a pre-teen of average height with short blonde hair and large emerald green eyes. He has apparently high altitude and seems to be very slim. His common garment consists of a pink coat with green trousers.
Despite being brothers, Kazuma and Ren have very little in common, though this is justified due to the eight-year age gap between them. While Kazuma is blunt, arrogant, selfish and apathetic. Ren is soft spoken, humble, kind and somewhat effeminate. These traits lead to many being protective of him and even fighting over him, including his best friends Kanon and Tatsuya, with the latter even worrying he'll become too feminine if he hangs around the former too much. He is also shown to be easily saddened when Ayano begins to insult Kazuma, even if she didn't mean to, such as when she states she couldn't even remember him despite fighting him for Enraiha.
Despite his kind and soft spoken nature, Ren is very brave and does not hesitate to help others when need be, and will gladly fight for his family and friends or when provoked or simply when it's necessary. However, he is a very merciful person and prefers to avoid killing enemies if he can help it, such as when he purifies Yuji Tsuwabuki's aura and change him back into his original form, rendering him unconscious but not killing him. Although Ren's appearance earns him some contempt for his male classmates, Ren nonetheless appears to be very popular at his school, and excels in his studies and athletics.
Ren has never liked the fact that Kazuma was cast out of the family simply for having no power, and to this day, still holds him in high regard. He always hopes that one day, his brother Kazuma and his father will get along someday so that the three of them can be a family again. Like Ayano, Ren also doesn't approve of Kazuma's cold attitude towards the Kannagi family, believing that they're still his family even if they have disowned him, and is shocked by his willingness to let them all be killed by the Fugas.
Ren looks up to both Ayano and Kazuma, though his opinion of the latter reaches near-worshipping levels. Despite this, he tends to act as the 'straight man' between the two, often expressing exasperation at their frequent arguing and general coldness towards one another. Just like Juugo, Yukari and Nanase, Ren is also shown to be supporting of Kazuma and Ayano's growing feelings towards each other, believing they would be good together if they just learned to get along.
Ren is first shown when he greets Ayano, and she makes upsets for her father when she sneaked in the past hour.
Ren begins to tell her that his brother has returned to Japan. Ayano tries to remember if Ren had a brother. Ren reminds her that his brother lost against her during the Enraiha inheritance ceremony. She remembers that she defeated him with one burst of magic. Later they both broke into a meeting with the clan boss, her father Jūgo Kannagi, Genma Kannagi and Shinji Yuuki. In the conference room, he learns that Kazuma is a wind magic user. Then he states that it is unbelievable that someone in the head family would become a user of the wind magic. The next day there is confusion in one of the branches of family households; three members of the unit were murdered. Hyoue Kazamaki claims that the victims were killed with the magic of wind stronger than his knights,
Despite his age and timidity, Ren is actually a very powerful fire magic user, his skills rivaling even that of Ayano, and then there's the matter of his intelligence, as seen when he turns sand into magma so that it cannot be used by Yuki.