This article contains major plot spoilers from the light novels that have not been revealed in the anime. Proceed at your own risk.
|Height||158 cm (5'2")|
|Birthday||May 15 (Taurus)|
|Likes||Pasta, citrus juice|
|School||Kitauji High School|
|Occupation||Student (first year)|
|Light Novel||Volume 1, Prologue|
Reina Kousaka (高坂 麗奈 Kōsaka Reina ) is one of the main characters of the Hibike! Euphonium series. She is a first year trumpet player who plays in Kitauji High School's concert band, continuing on the same instrument from elementary and middle school.
Reina is a teenage girl with average height and build, described to be "very beautiful" in the light novels. She has long, straight black hair which reaches down below her shoulders, which she sometimes wears in a ponytail. She has blue eyes and a light complexion, and is usually seen wearing a normal Kitauji High School uniform for first-year female students.
Reina is a calm and quiet girl who is very individualistic and passionate about music, and it is on principle that she does not attempt to be friends with someone who doesn't interest her.
She herself becomes upset with others when someone talks ill about Noboru Taki, whom she has a crush on for which people sometimes comment on her. Though she has a polite manner, especially to her seniors, she is told that she is not taken so well by others because she usually wears a surly expression.
Reina is also portrayed as somewhat mysterious, only opening herself up to her closest friends. She is at times standoffish and can often speak very bluntly when it comes to her pride as a musician. Because of this she has had trouble making friends in the past. She has an appears to be distant and emotional, focusing solely on her drive to succeed.
However, as Reina grows closer to Kumiko, she begins to let her guard down and a different, friendlier side of her personality is revealed. After becoming friends with Kumiko, she grows close to her and supports and cares for Kumiko deeply. She shares with Kumiko her intentions to become truly special, which is why she plays the trumpet. She claims that with her trumpet, she will to become her own kind of special.
Throughout the series, Reina appears to grow more empathetic, as she apologizes in later episodes for her rude behavior for the first time on screen. Later in the series, Kumiko mentions that Reina has changed, as she is willing to become friends and interact more with other people such as Hazuki and Midori. 
Reina has been playing the trumpet since she was very young, and continues to do so at Kitauji High School.She went to the same middle school as Kumiko and was a member of her junior high school's concert band along with her. In their time in middle school, Kumiko noticed Reina crying that left the band slightly short of qualifying for the next round of advancement and believed her to be crying out of joy. However Reina was actually dismayed that she had lost her last chance to go to the nationals as a middle school student. She then ran away sobbing, leaving Kumiko feeling speechless and guilty.
It is also known that Noboru Taki's family and Reina's family knew each other before the start of the series, so Taki-sensei has known Reina since she was little.
She also cherishes her trumpet dearly, which was given to her by her parents when she was a junior high school student.
On her first day on Kitauji High School, Reina decided to join the band to play the trumpet, which made Asuka happy and she warmly welcomed her. However, since Kumiko (along with Hazuki and Sapphire) were present and heard her admission, Kumiko became very shocked to see Reina again, especially after the incident she had with her in junior high. Throughtput the series, while Kumiko has been acting very awkward around Reina ever since meeting her again, Reina hasn't really acted in such a way. Following some events, Reina grows closer to Kumiko and they eventually becomes friends. With Kumiko by her side, her motivation to become a better trumpet player has increased.