Shiina, given name unknown, otherwise referred to by his nickname Shii (しー), is an adult friend of Shuichi and Yoshino. Eventually the boyfriend of Hiroyuki Yoshida, he is portrayed as a carefree and understanding person who first took interest in Hiroyuki in high school, when Hiroyuki was teased and bullied for being a male to female transgender teenager. Shiina has always been encouraging, even when the chips were down. Shiina's strong sense of justice, and fair play, caused him to step up and help to protect Hiroyuki during a day when the bulling was especially bad. The two shipped together for the rest of high school. After graduation, Shiina's loving concern of Yuki prompted him to find out that Yuki has transitioned to living as a male-to-female transgender woman. He generally just watches over Shuichi and Yoshino, along with Yuki, but he sometimes does bold and unexpected things.
Shii's Story (Anime)
When Shiina was very young, his two older sisters would dress him as a girl, outfitting him with pretty dresses and such. Shiina always knew that he was a male, but went along with the cross dressing to please his sisters. Gradually, the two sisters lost interest in doing that, and drifted away from Shiina. He was Hiroyuki's classmate in elementary school, but the two never interacted much until they entered the same high school. Not many details are given about him, save that he is very caring towards Yuki and has been with her since the beginning of her transition to her correct gender of female.
One day at high school, Hiroyuki was bullied so badly, that Shiina knew that he had to do something. He was the only student who tried to protect and help Hiroyuki, when all others jeered or looked the other way. Alone in the hallway that day, Shiina saw the mocked, vilified, and humiliated Hiroyuki standing alone and afraid, and he knew from that moment on that they were going to be together for the rest of their lives. It is stated that he was the only one who accepted Yuki for who they were, a teenage female, even before Hiroyuki underwent the transition from male to female. After graduation from high school, a concern for what happened to Yuki prompted him to find out about her at the bar and night club that she now owned and managed. From there, his relationship to Yuki deepened to that of boyfriend and girlfriend. Soon, they began sharing an apartment together. Unlike Yuki, Shiina had many friends in school, but gave all of that up to be with Yuki.