Rena Kariu (狩生 玲奈, Kariu Rena) is Arata Kaizaki's classmate who sits beside him after he became a teenager. She is a very stubborn and competitive girl, and is a member of the girls volleyball club at Aoba High School.
Rena is a well-developed girl with a slim and average build.
She has brick colored hair that is tied into a single ponytail on the side in a grey ribbon while her hair curtains her face nearly covering her blue-grey eyes.
She is mostly seen wearing her school uniform though she does wear her gym clothes while practicing for volleyball.
At first, when Arata Kaizaki meets her, Rena is quite judgemental considering how on the first day he was caught with cigarettes in his bag, but she is considerate enough to lend him a pencil and eraser when he needed.
She is also shown to be quite determined as she came to the first day of school, sick, in order to beat Chizuru Hishiro in the exams and become a student council rep for the class.
Rena also has trouble expressing her feelings at times and will consider avoiding them or trying to cause others the same misery she's feeling such as when she tried to steal Chizuru's bag.
In middle school, Rena watched Honoka Tamarai play volleyball and soon accused her of holding back. After several taunts, Tamarai reluctantly smacked the volleyball straight for Rena; knocking her to the ground.
On the first day of school, Rena notices Arata Kaizaki panicking after realizing he had forgotten his pencil box. She soon hands him a spare pencil, annoyed that someone his age would easily forget something so simple.
After the tests, Kaizaki thanks Rena for helping him as the two begin to talk. It is soon revealed that Rena has a slight cold, in which Kazuomi Ōga recommends that she should have stayed home. However, Rena states she is perfectly fine and wants to pass the exams in order to achieve the spot on the student council representative and receive the class pin.
Kaizaki Arata - Kariu's first impression of Kaizaki is that he was an idiot (because he brought cigarettes instead of a pencil case on the first day) but as the story progress, she was able to be friendly to Kaizaki as they continue to interact.
Honoka Tamarai - Rena and Tamarai have been friends since junior high after Tamarai transferred in order to start fresh.
Hishiro Chizuru - Rena has considered Chizuru her academic rival since the beginning of the school semester, after losing to her during the exams. She soon begins to loathe the latter as she notices Chizuru hanging around Ohga a lot more while the two tend to the student council work. The two later become friends after Rena nearly steals her bag and apologizes after being caught by Kaizaki.
Kazuomi Ohga - Kariu has had feelings for Ohga since their time in middle school. She even had matching piercings with Ohga during this time. During the summer vacation, she decides to confess her feelings to Ohga but he confesses his feelings first. After being bashful for a while, they decide to become a couple.
- "I can't help being competitive. I always want to be first."