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Essentially, he is the only person Youji can "happily" end up with. He has 4 of the 6 endings with Youji. Initially, he is cold to Youji, and Youji dislikes him and finds him creepy. However, Youji realizes Tetsuo is the only person he can relate to and Tetsuo is always there for him. He is the one who really understands and cares for Sakiyama the most, making him the perfect person/mate for Sakiyama. He is very stoic and aloof, more so then Youji, yet very sweet and caring. He is also able to calm Youji and dispel his discomfort. (though anonymously at first) Whenever Youji gets another of his panic attacks, Shironuma seems always to be able to calm him down.
He does rape Youji at one point in the game and, whilst walking Youji home, murmurs "Sorry for being so rough last time." (「この間は……乱暴にして、悪かった」). Then walked away.

Sakiyama and him have similar childhoods. He lives with his step-parents.
His route ending songs are:
Miracles May (Violin Version) by Ito Kanako
Miracles May by Ito Kanako
Diving Deep by Pale Green
The Red Road by Pale Green