Nagasaki Soyo is a first-year student at Tsukinomori Girls' Academy and the bassist of MyGO!!!!!.

Soyo's original surname was Ichinose. While in elementary school, she moved out with her mother and adapted her mother's surname Nagasaki. Due to her mother being busy with her work, Soyo had to spend a considerable amount of time alone.

Soyo was the bassist of CRYCHIC. She met Sakiko after the latter approached her after a brass band's concert that Soyo played in. Though she is good at hiding it, the band's disbandment actually hit Soyo hard, as she not only had taken Sakiko's words about peoples' fates being one when they're in a band together literally, but also because she couldn't understand why they should break up when they were having so much fun every day. She also felt that it was one of the first things she had done out of her own will, as she started attending Tsukinomori out of her mother's insistence and joined the brass band only because her classmates did the same.

After the girls started high school, Soyo started becoming more desperate, leading to her attempting to contact Sakiko multiple times afterwards because she wanted the band to start over.

Soyo is a very kind and mature girl and she is often compared to an older sister. Due to this, she is often relied on by her peers. She has a habit of fiddling with her fingertips when she's thinking or planning something. Although not outwardly hostile, Soyo can be quite cunning at times.

Deep down, Soyo has a hard time letting go of the past and moving forward, especially in regards to her former band. A part of her selfishly clung to the idea that CRYCHIC still hadn't actually broken up, as shown in the way she reacted after MyGO!!!!! decided to play Haruhikage during their first concert. As Sakiko kept rejecting her attempts to revive the band, Soyo cried that she had worked so hard for CRYCHIC, and even claimed that she is willing to do anything for the former. In reality all of this stemmed from her not wanting to be left all alone.