She is a first-year trumpet player who plays in the Kitauji High School Concert Band, continuing on the same instrument from elementary and middle school. She was also a member of the concert band club at the same junior high school as Kumiko Oumae.
Reina is a teenage girl with an average build and a considerably short height, described to be beautiful in the light novels with a light complexion. She has long, straight black hair which is about lower-back length, which she sometimes wears in a ponytail. She has two long strands of hair laid on her shoulders and two smaller strands framing her face and purple eyes.
She 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. Although 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. She can be somewhat cold and upset with others when someone talks ill about Noboru Taki, whom she has a crush on for which people sometimes comment on her.
Reina is portrayed as a somewhat mysterious girl who doesn't open up to people easily, which is why it is on the principle that she does not attempt to be friends with someone who doesn't interest her. 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 appears to be distant and emotional, focusing solely on her drive to succeed.
However, as Reina grows closer to Kumiko Oumae, 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 become her own kind of special. She cherishes her trumpet dearly, which was given to her by her parents when she was a junior high school student.
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 Katou 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 Oumae 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 when Kumiko asked her if they could really make it to Nationals, leaving her 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.
However, since Kumiko Oumae (along with Hazuki Katou and Sapphire Kawashima) 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.
Throughout 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 become friends. With Kumiko by her side, her motivation to become a better trumpet player has increased.
- The name Reina means "beautiful, lovey" (麗) (rei) and "apple tree" (奈) (na).
- Reina's surname Kousaka means "tall, high" (高) (kou) and "slope, hill" (坂) (saka).
- According to the official Sound! Euphonium novel website:
- Her favorite colors are blue and white.
- Her hobbies are watching movies by herself and going to concerts with her father.
- Her special skill is playing the trumpet and piano. She started piano at the age of five.
- In the light novel, Reina is the Representative for the entering students also known as Freshmen.
- In the light novel, Reina is known to have good grades and is a very good student.
- In the light novel, before giving her Representative speech, Reina and Kumiko Oumae meet eyes for a few seconds.
- Reina takes private trumpet lessons outside of rehearsals.
- Reina likes pasta, citrus juice while she dislikes natto.
- Additionally, Reina is also the instrument manager.
- Reina's trumpet is probably based on Yamaha YTR-6310Z model.
- Reina is determined to go to music school. 
- Reina's father raises and sells fish.
- Reina Kousaka on Hibike! Euphonium Archive
|Kitauji High School Concert Band during Kumiko Oumae's first year|
|Note: Section leaders are shown in bold text
1Rejoined the band
2Left the band
3Also in charge of the Tuba and Double Bass sections
|Kitauji High School Concert Band during Kumiko Oumae's second year|
|Note: Section leaders are shown in bold text|
|Kitauji High School Concert Band during Kumiko Oumae's third year|
|Note: Section leaders are shown in bold text|