So, this is in a bsd BEAST plot setting, and I won't do him in a normal Bungo Stray Dogs world setting because I made him to be in BEAST and not anywhere else (i'm working on the imagery trust the process).

• Reiji Tsushima, younger brother of Osamu Dazai or by his real name Shuuji Tsushima, is a man of his word, even after death.

[...] Reiji always looked up to his older brothers and wanted to be like them; he also wished to be a painter and illustrate Shuuji's works with his own one day. [...] But illness got his life a fiew monthes before his 18th birthday.

Reiji appears as an hallucination of Dazai/in his nightmares, and he's only shown through Dazai's eyes, so only how Dazai saw him through his interractions with his younger brother. (wich, in fact, was far from what he was really.)