Shiina, also known by the nickname Shii-chan, is an adult character and the boyfriend of Hiroyuki Yoshida, later known as Yuki. Shiina first noticed Hiroyuki during high school when Hiroyuki faced bullying as a transgender teenager. During a severe incident, Shiina stepped in to shield Hiroyuki, driven by a strong sense of justice and equality. This marked the start of their close relationship, which continued throughout high school.

After graduation, Shiina remained concerned for Yuki's well-being. He eventually discovered she had transitioned to living as a woman. Shiina sought her out at the bar and nightclub she owned, leading to a deepening of their romantic relationship. The couple subsequently moved in together, sharing an apartment.

Shiina is portrayed as carefree and understanding. His childhood included experiences where his two older sisters dressed him in girls' clothing, such as pretty dresses. He always identified as male and participated primarily to please them. His sisters eventually lost interest in this activity and drifted apart from him.

In interactions, Shiina generally assumes a supportive role, watching over the younger characters Shuichi and Yoshino alongside Yuki. While typically observant, he occasionally engages in bold or unexpected actions. His commitment to Yuki involved significant personal sacrifice, as he distanced himself from his broader social circle to prioritize their relationship.